Bower.json (closes #39)

This should make `bower.json` the default everywhere without breaking packages that use `component.json`. Packages that still use the old name cause a warning to be emitted on stderr.

The code for the fallback cases is relatively contained, so should be quite easy to remove when we no longer want to do that.

Feel free to suggest more test cases. Bear in mind that most general usage should be covered by the existing tests.