Extracted "Proxy Source Collection" from PostsBundle
Continued extracting functionality from PostsBundle

Filters and Maps are now built into Sculpin Core. A few helper filters and maps
are provided (extracted from PostsBundle) including `AntPathFilter` and
`DefaultPermalinkMap`.

PostsBundle now has a compiler pass. Additional maps can be added via
configuration by tagging services with `sculpin_posts.posts_map`.

The `ProxySourceCollectionDataProvider` is now using `FilterInterface` and
`MapInterface` to determine which `SourceInterfaces` should be included in the
collection.
Added a note to the configuration about order being important.
Merge pull request #78 from simensen/proxy-source-collection

Extracted "Proxy Source Collection" from PostsBundle