Add support for different directory structure

[nelsonsar]

Some packages do not are distributed having, for example, "src" dir. I patched Onion to support that on my fork at: https://github.com/nelsonsar/Onion.

Basically I added a new entry on the ini file named 'structure' and used a notation:

role=lib,src

[c9s]

Hi! Thanks for your patch,

Can you send me a pull request, and maybe we can merge it from your changes. :-)

[nelsonsar]

Yep, for sure I'll send it to you! Nice job on Onion btw :-)

[c9s]

Thanks ;-)