Introduce the initial refactoring module

See http://xwiki.markmail.org/message/e7udv44er33biv3f

The refactoring module consists of a set of components that will allow refactoring a wiki in many ways including:

• Split an existing document into multiple documents
• Merge documents into a single document
• Search and replace words in a document
• More..

Initially document splitting support will be introduced, later refactoring module will be extended support other operations. Also this task includes finalizing the design / public API for the refactoring module.