Full semver support in Bower #1

@devoncarew 

* Support for (^|~|>|<|>=|<=)1.2.3 in dependencies in bower.json with version resolution via GitHub tags.
* Support `latest` as a meta-tag designating the latest released tag + default an unspecified tag to `latest` instead of `master` previously.

A bunch of less important stuff is still to be implemented: will file individual issues (e.g. use "resolutions" from bower.json instead of proprietary overrides from CDE flags, resolve possible conflicts, use Bower registry to discover packages without explicit paths...).