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

[vattay]

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.

[coveralls]

Coverage remained the same when pulling 9bca02b on 3martini:merge-dot-bowerrc into b1cc827 on 42dev:master.

[SergeyKishenin]

Seems like we should create .bowerrc at the project's root manually to make this feature work, right? It's not the preferred way, I think. Have you tried to run tasks if there is no .bowerrc there?

[coveralls]

Coverage remained the same when pulling fa2c7c1 on 3martini:merge-dot-bowerrc into b1cc827 on 42dev:master.

[vattay]

You were right, I forgot that a .bowerrc is optional. Added a check if the file exists.

Solution is a bit ugly since I have copied the read_bowerrc function in both the dsl.rb and bower.rake, do you think a static utility class is called for?

[rharriso]

looks like pull request #41 created some conflicts. Please resubmit pull.

[coveralls]

Coverage remained the same when pulling 556bd8a on 3martini:merge-dot-bowerrc into 67f9f1f on 42dev:master.

[vattay]

@rharriso I merged the changes and rebased, should be good for a clean pull.

[SergeyKishenin]

Conflicts @ Changelog.md now. Rebase again, please.

[coveralls]

Coverage remained the same when pulling bcb0416 on 3martini:merge-dot-bowerrc into 8e410af on 42dev:master.

[SergeyKishenin]

Thanks, merged!