Full semver support in Bower #1

Support for (^|~|>|<|>=|<=)1.2.3 in dependencies in bower.json with version resolution via GitHub tags.

A bunch of less important stuff is still to be implemented: will file individual issues.
No-op changes
Comments/UI strings
Address review comments
Merge pull request #3615 from ussuri/bower-semver-support

Full semver support in Bower #1