default layouts per path and type

> This adds the feature requested in #453.
> 
> While I'm aware this is still under discussion, I must say I'm strongly for it because it eliminates a lot of duplication. All jekyll sites I've created so far have reused one or two templates over and over - that would be much easier with this ability.

# Update
This implements a more complex feature where default layouts can be set per path (and type) in `_config.yml`, as suggested by @mojombo in [#453](https://github.com/mojombo/jekyll/issues/453#issuecomment-16356932).