Allow definition of multiple source directories

I made it possible to add multiple source dirs.

Main points:
- `stdTestSourceDirectory` is needed to specify a default value if one doesn't specify anything. This is ugly and it is possible to set this value from the configuration. Any ideas on how to fix this?
- No @required attribute for source directories anymore. If one doesn't specify anything, nothing is done.
- I didn't came up with a test case. Any ideas on how to test the new behavior?
- Probably the SBT plugin and the command line tool needs to be updated too. I didn't look at them yet (documentation of course as well).

Do you like the changes? Maybe you have an even better idea on how to realize the definition of multiple dirs.