Put SBT plugin in main build

- The SBT plugin is now in the main build
- Change the root project to compile:
  - library
  - compiler plugin
  - sbt plugin
- Shared settings for travis should be OK
Travis build + starting updating Symbol.source

- Travis build should work for the three components (Library, plugin, sbt
plugin).
- Update of the demo main file to use mirrors
- Update of the ToolBox in consequence

TODO: figure out how to link the source file. Associated file and the source
associated with position seem to be empty using the mirrors.