Git DB API: Implement the Tags API

Fix Issue #131. Git DB API: Implement the Tags API.  This is my maiden contribution so please let me know if there is anything I should have done differently.

Please also take a look at `Ocktokit\Models\Response\Tag.cs`.  I have used a `DataMember` attribute to map the "tag" json response property to the `Name` property of my `Tag` class.  I was unable to work out how I could test if this attribute would work (other than reading the json serializer).  Apologies for that.  

Please also let me know if there is anything else I can improve upon.

Regards