Added a gitVersion property to use different maven and git versions

This patch allows to specifiy another version or branch in the git repository than specified with the dependency.
```
compile('org.batcha.gradle.plugins:git-dependencies:0.1') {
    ext.git = 'https://github.com/bat-cha/gradle-plugin-git-dependencies.git'
    ext.gitVersion = 'some/otherBranch'
}
```
Since I'm fairly new to gradle I don't know if this is redudant and the functionality was already available elsewhere.  This however solved my problem with a discrepancy between maven and git version for the moment ;-)