add bower.json

fixes #225

Looks like the package is already registered on bower, but not having a bower.json makes bower install too much stuff. (also not having a bower.json in your source makes people think that the package isnt on bower)

This bower.json will ignore some stuff that you wouldnt need - the `test` and `vendor` dirs. 

I am not sure if you will need to create another tag in order for bower to notice this json - easy to test.