Read modules's main file from package.json 

Hello!

Thanks for the work done on this grunt task, I found it great for my needs. I just went through a small issue in my pov.

### My issue
I just realised - when trying to incorporate modules that had different directory names than their main file (i.e. ```backbone-amd``` which has ```backbone.js``` as main file) - that it was not included in my rjsconfig.

### My fix
This pull request tries to read the ```main``` attribute from the potential ```package.json``` file of a given module to find the main file to use.


Please let me know if it's something that you think would be interesting to incorporate in this project.

Cheers,
Paul