Spec improvements

[dometto]

Suggestions by @SkyCrawl:

• Things like Gollum::BlobEntry should be moved from gollum-lib to adapter_specs (gollum-lib should require and use an adapter, just like it is now, but all git-related types and methods should be defined there),
• I imagine it would be a lot better if adapter_specs defined fully documented communication between underlying git layer and gollum-lib (types and module methods), instead of some "vague descriptions" (if possible, of course). In other words:
  • gollum-lib would only reference methods and instance variables defined in adapter_specs and its types,
  • all methods of adapter_specs should only define arguments and return objects of types predefined in adapter_specs (or primitive types),
  • all methods that require custom implementation should have their skeleton defined (and documented) in adapter_specs.