tag objects from references

It looks like it is currently impossible to access the Tag object from a Reference that is a tag.

The C API provides git_reference_is_tag and git_tag_lookup, but there doesn't seem to be an equivalent functionality in pygit2. Searching for git_tag_* functions in src/ suggests that there are no hooks for the git_tag_* functions.

Currently Tag is a subclass of Object. This doesn't really make sense. Unlike Blob, Tree, and Commit a Tag is a reference to a Commit. Also, the docs for git_tag_tagger leans toward Tag.Tagger object to just be of type Signature instead.

It makes sense to make Tag a subclass of Reference similar to Branch, remove Tag.Tagger (and replace with the already existing Signature, changing Repository.create_tag to match), and have Repository.References.get() return a Tag object where appropriate.

Beginne mit der Untersuchung der vorhandenen Hooks in src/ und der Einstiegspunkte Repository.References.get() und Repository.create_tag. Vergleiche sie anschließend mit git_reference_is_tag, git_tag_lookup und den anderen git_tag_*-Funktionen. Die Arbeit ist abgeschlossen, wenn Tag-Referenzen ein Tag-Objekt offenlegen, Tag dem vorgeschlagenen Reference-Modell folgt und die Behandlung von tagger durchgängig Signature verwendet.