Support for next/prev post within the same category.

Use like this:

{{post.next(true).title}} or {{post.next(true).permalink}}
Merge branch 'patch-1' of github.com:slimndap/timber into slimndap-patch-1

* 'patch-1' of github.com:slimndap/timber:
  Support for next/prev post within the same category.
wrote test for prev/next post by category, adjusted syntax to support other taxonomies in the future
Merge branch 'master' of github.com:jarednova/timber

* 'master' of github.com:jarednova/timber:
  Fixes Issue #156