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.

Start by inspecting the existing hooks in src/ and the Repository.References.get() and Repository.create_tag entry points, then compare them with git_reference_is_tag, git_tag_lookup, and the other git_tag_* functions. Done means tag references expose a Tag object, Tag follows the proposed Reference model, and tagger handling uses Signature consistently.