Makes Zencoder path configurable.

It's now possible to configure the destination path of files
processed by zencoder in the pageflow initializer.
Merge the path as the key `:path` into `zencoder_options`, for example:

    config.zencoder_options.merge!(
        [other options]
        :path => "/:zencoder_asset_version/custom/part/:id_partition/:filename"
    )

Paperclip interpolations apply.
Database Seed DSL

Replace procedural `pageflow/seeds` file with `Pageflow::Seeds` mixin
that provides a DSL to create seed records.
Merge pull request #71 from kioopi/zencoder_path

Makes Zencoder path configurable.
Merge pull request #73 from tf/seeds

Database Seed DSL
Update CHANGELOG.md