Documentation fixes

* Add Sensio Sphinx extensions
* Update Sphinx config and added Makefile
* Exclude docs/_build directory
* Reword introduction
* Reword and small tidy-up of overview
* Corrections to installing
* Small tidy up of configuration
* Tidy up caching
* Very small changes to metadata
* Fix code examples and tidy up of input
* Tidy up and format type
* Fixes for Doctrine ORM
* Tidy up WhereBuilder
* Tidy up internals
* Formatting for class names
Fix typos and add XML filtering to list.
Add brief instructions for building documentation to README.md
Merge pull request #22 from StuBez/documentation

Documentation fixes