Windows doesn't handle dynamic allocation well.

- transaction.cpp
  - explicitly define est. doc max size of 900.
  - No need for string since we're stating size.
- message.cpp
  - let string handle doc size allocation.

AUnit has stopped correctly building tests for the partially-supported ESP8266 platform.
It seems this may be a change in ESP8266 Core, so this PR disables ESP8266 tests in PlatformIO until support is worked out.

- Further decouple OS code from IoT code.
- Add missing ESP8266 functionality.
- Allow removal of lib/STL dependency.
  - STL lib is only included to provide a conv. method for IoT to use date.h
  - date.h is now only used by OS builds, which will have the STL by default.
- update changelog.
- add PIO filters for lib/date.

The Arduino IDE requires the use of a single true tab separator between the keyword name and identifier. When spaces are used rather than a true tab, the keyword is not highlighted.

Reference:
https://github.com/arduino/Arduino/wiki/Arduino-IDE-1.5:-Library-specification#keywords

Current tests are ran in different orders per platform.
This led to a modified global test custom_network being modified improperly.

This PR properly handles that order variation by creating a local object to be modified.

In future iterations, an AbstractNetwork will allow `const`.

Current implementation sometimes fails to properly serialize numbers in the `toJson()` method.
This is especially detrimental to some Linux builds and all Arduino IDE builds (Arduino IDE Library version (Cpp-Crypto-Arduino-v.0.3.0)).

Specifically, this PR does the following:
- moves `<cinttypes>` to `crypto_helpers.h` for non-IoT builds.
- forces type serialization for:
  - `amount` (llu/uint64_t).
  - `fee` (llu/uint64_t).
  - `network` (uint8_t/int).
  - `timestamp` (lu/uint32_t).
  - `type` (uint8_t/int).
  - `version` (uint8_t/int).
- updates the changelog.

The current implementation only allows use with Devnet unless values are manually changed in `/src`.

This PR enables network configuration via the public api.
(Devnet, Mainnet, Testnet, and Custom Networks (bridgechains).

Specifically this PR does the following:
- Creates a Configuration class.
- Creates Fee and Network Manager children for the Configuration class.
- Allows passing a `Configuration to` Transaction Builder with a default value of `Devnet` & `StaticFees` (non-breaking).
- Adds a FeePolicy-type.
- Adds a Fee class (container for fee policies).
- Adds a Networks class (container for preset networks).
- Updates the TransactionTypes enum.
- Improves the `Network` class and Network implementions.
- Updates the `Slot` class to reflect changes (non-breaking).
- Updates Transaction-related classes to reflect updates (non-breaking).
- Adds tests for all changes.
- Updates the Arduino IDE script.
- Updates `.ino` Arduino sketches with Configuration examples.
- Updates documentation with Configuration examples.
- Updates the `keywords.txt` file to reflect changes.
- Updates the changelog.
- Updates version to `v.0.6.0`.

Some small changes are technically breaking-changes.
(e.g. `Devnet` vs `Devnet()`, and `Fees` vs `Fees::StaticFeePolicy`)

…ge-changes-at-105