RestDocumentationResultActions->RestDocumentationResultHandler

Previously documentation was handled by using a ResultActions
implementation that had custom methods added to it.

Now documentation is handled using a ResultHandler. This has a few
advantages:

- Fit better with the MockMvc programming model. This is similar to
  andDo(print())
- Support for using alwaysDo
Update the README to reflect the changes to how document is used
Merge branch 'documentation-resulthandler'

Closes #3