fixes #291

have type declarations that work with TS v3.8, and above

down to which version of typescript should the type-declaration inter-compatibility reach?
TS 3.8 it is the latest version of the previous major version. According to TS' version policy v4 would be a breaking change, so v3 users could upgrade their projects to v3.8 without problems and therefore have full access to type declarations of this very project.

background:
TS 3.8 appears to be the lowest typescript version i could get support for out of the box.
I might go below, by the use of https://github.com/sandersn/downlevel-dts(untested).

• render dedicated typedefs
• apply type defs, instead of sources
• have a CI example to test the compatibility with 3.8, and 4.X