added goal 'compile-directory'

Dear Protostuffers, 

for some of our projects we needed the functionallity to compile all proto files in a certain directory(ies). So I decided to add a maven mojo, which provides this functionality, binded to a new goal. 
The new existing goal is called "compile-directory". I find the changes really useful and I hope that you'll integrate them into the project. 
At this point, thanks to your effort creating protostuff - it is an amazing project.
If there are any questions about the code, I will try to answer them.

Best regards - D. Häberlein, TIQ Solutions