add mobile poster image feature for video and background_video pages
Make Zencoder URLs interpolation pattern configurable

Pass `:url`, `:hls_url` and `:hls_origin_url` from `zencoder_options`
set in initializer to `ZencoderAttachment.default_options`.

Allows to override the interpolation pattern used to construct urls to
Zencoder generated files. Previously only available for the :path
option.
Use hls_origin host in m3u8 playlists
Do not prefix relative URLs with Default Protocol in Zencoder Attachment
Update Changelog
Update Changelog

Add note about manual registration of audio loop page type.
bump version to 0.5.0
Merge branch 'master' of https://github.com/ablotny/pageflow

Merge mobile_poster