ignore generated Doxygen documentation
Doxyfile: prepare for Markdown rendering
rapidjson.h: add some file-level doxygen documentation

Although not yet complete, the rapidjson.h header contains most of
the customisation macro definitions.  These should be documented via
Doxygen as well.  This requires adding file-level documentation.

Some parts are excluded via `@cond` to avoid cluttering the documentation
with internals too much.

Secondly, a brief description is added to the 'rapidjson' namespace to
include the namespace-level elements (enums, typedefs, functions) to
the generated documentation.
GenericMemberIterator::Type -> GenericMemberIterator::Iterator

Rename nested typedef to avoid confusion with JSON Type enum.
GenericDocument: improve Doxygen documentation
Doxygen: document some UTF-8 typedefs
(Pretty)Writer: improve doxygen documentation
GenericReader: improve doxygen documentation
readme.md: prepare Markdown for GitHub and Doxygen rendering
features.md: prepare Markdown for GitHub and Doxygen rendering
tutorial.md: prepare Markdown for GitHub and Doxygen rendering
stream.md: prepare Markdown for GitHub and Doxygen rendering
Doxygen: fixup main page

To avoid the {#mainpage} tag to be rendered on GitHub, explicitly add a
`\mainpage` section to `rapidjson.h`, referring to `readme.md`.
Merge pull request #49 from pah/doc/doxygen-markdown

Integrate Markdown pages into Doxygen documentation