Fetch tags

This pull request addresses https://github.com/matteosister/GitElephant/issues/55
That's valid and I can change the param to work that way if you want, but I modeled my changes to FetchCommand based upon looking at RemoteCommand, which accepts an $options array.  It seems to me that the $options command (though admittedly, less user friendly) is more versatile as a general convention, since so many git commands can accept a slew of options which would be inconvenient to accept as individual boolean params.

Anyway it's your baby, I'll go whichever way you think is best.