When users of the programmatic API update the version, they may do so without having to inspect the new contents of the bower.json file to find out the new version.
This is useful in scripts where the tag needs to be pushed programatically; the user can push the new tag explicitly.
This commit changes the interface of the command functions exported by the
files in `lib/commands`. The functions now return a promise and accept a logger
as the first argument. This has several advantages
* The promise style is consistent with the rest of the code.
* It removes a lot of duplicate code.
* The command factory does not need to proxy the logger object.
