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.

…#105)

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`.

The current library manifest uses a wildcard `*` for the `platforms` value incorrectly signifying that it supports all platforms.

This is incorrect as the Cpp SDK's only officially support the espressif8266 & espressif32 platforms.

This PR corrects this discrepancy.

…master