When installing via "dev" or "git", check haxelib.xml for dependencies

Also, allow "haxelib git gitpath branch subdir" syntax.

The checks for haxelib.xml will obviously have to change once we switch
to JSON format.  I've just included a patch here that I had previously written.

Once we're stable on the JSON stuff I'll update this.
Merge branch 'master' into feature/GitDevInstallDependencies
Change xml checking to json
Minor fix so "getCurrent()" returns dev if dev is being used
Fixes and better error handling for installing dependencies of dev package
Merge pull request #16 from jasononeil/feature/GitDevInstallDependencies

When installing via "dev" or "git", check haxelib.xml for dependencies