Integrate Markdown pages into Doxygen documentation

This pull-request is an initial try to integrate the new Markdown documentation into the Doxygen documentation.  The rendered result should be available at http://pah.github.io/rapidjson-upstream soon.

Note, that this now requires to build the Doxygen documentation from the top-level directory (added to `readme.md` as well):
```
doxygen build/doxyfile
```
One option might be, to move the Doxyfile to the root-directory.

The required changes to the Markdown files are needed to enable correct rendering both in Doxygen and on GitHub (code blocks, image links, etc.).

Additionally, some Doxygen comments have been improved, e.g. by enabling documentation of the namespace-level functions and typedefs, etc (5174fe4) as well as more information on GenericDocument, and Reader/Writer.

In order to avoid (Doxygen) confusion between the JSON Type enum and the (nested) typedef `Type` in the member iterator, this typedef has been renamed to `Iterator` instead.