Initial commit for contribution of documentation.

+ Updated `.gitignore`
+ Added `Doxyfile`.

[Doxygen](http://www.doxygen.org) has been setup to look for documented `C` fles within:

+ `src/sodium/includes`; and
+ `test`

When run with the Doxygen tool documentation will be generated within
the directory `doxydoc`. The projects README.markdown file is used at
the documentation's main page.

More information on Doxygen tags and abilities can be found on the
projects website: http://www.doxygen.org
Added api documentation.

The outward facing api has been documented where possible. If the
documentation has not been added a `@todo` annotations has been
added. Doxygen will constuct a list of TODO's automatically.
sodium/version.h.in: fix pasto