Use root .bowerrc as starting point, rather than creating one from scratch

In the case where a custom .bowerrc is needed, for example, with custom registries, it seemed that bower-rails was ignoring .bowerrc in the rails root directory, and generating a new .bowerrc per group. This loses any required settings. I didn't see any other way to set registries in bower-rails.

This PR reads the root .bowerrc, and merges in the "bower_components" directory to create the new generated .bowerrc files.