Splits editor/base.js in base.js and include.js

Projects have to include both files in their editor.js

This makes it possible to include any new PageTypes before
the EditorApi is instantiated.

Example:

//= require i18n
//= require i18n/translations
//= require pageflow/editor/base <- everything to register pageTypes
//= require pageflow/external_links/page_type
//= require pageflow/editor/include  <- instantiates pageflow.editor
//= require pageflow/external_links/editor
Editor API: adds api for file selection.

Editor extensions can register handlers to re-use the file-selection
view to get files to use in custom models/new page types.

The files view can be invoked via API(pageflow.editor.selectFile).
Register models in engines to be associated with an entry/revision.

Authors of extensions/new page types can use this API to
register active records models to be associated with entries.