init: split ongoing..
BasePlugin: Better usage.
BasePlugin: give better report on unexpected error.

Propose to submit bug report with traceback and versions and arguments provided.
clean: x.__class__.y <=> x.y ; when y hasn't been rebound on the instance.
Merge branch 'BasePlugin_refactorization'

Begin of a refactorization and clean of shinkenplugins package.
For now only

- a split of previous classes/things to different modules has been done.
- a slight clean of them.

A rewrite of the BasePlugin will be tried later.