Allows the page_size value to be passed in the request.

It is a frequent requirement that API clients can indicate the number of
results that they require, rather than the API providing a fixed number
of results.

This change allows resources to specify a page_size_parameter which is
used to indicate the name of a query parameter which indicates the
number of results to return.

* The default behaviour remains unchanged.
* Allows each resource to specify the page_size_parameter, which is
  handled by the factory and assigned to the controller.
* Adds a pageSizeParameter to the ResourceController, which is used
  to extract a query parameter from the request; if not provided, the
  pageSize is used instead.
Merge pull request #87 from elyobo/configurable-rows-per-page

Allows the page_size value to be passed in the request.
[#87] Added unit test for page_size param

- Added test showing that page size parameter can be passed, and will be
  honored.
[#87] Added docs for page_size_param
Merge branch 'hotfix/87'

Close #87