Merge pull request #16 from joncotton/deprecations

#16 more sensible names, raise deprecation warnings, testing in Python 2.6
Initial work on a new Backend that allows Models to define their own template finding method.

This is similar to how South uses `south_field_triple()`; if the model supplies that method, it'll use it, otherwise it'll do the normal template lookup.

BasicLayoutBackend is functionally unchanged.
Move and unify tests to cover both new Backends.
New feature - can now specify a different base template directory.
Introduce mixins primarily for use with the new ModelProvidedLayoutBackend. Add one of these to a model for easy template lookup by `slug`, `full_slug` or `type`.