Introduction of a plugin-api module

... to allow decoupling of tatami and tatami extension

This is a prototype. Discussions are welcome
Main problem : introduce a new module without breaking the way people build and run Tatami and without breaking not-yet submitted other contributions

Introduced to be able to embed tatami-bot into Tatam
( a working prototype of embedded tatami-bot based on this pull request exists )
Note : this will induce a dependency to plugin-api.jar into tatami-bot