Add support for Doxygen

This includes files configured to build doxygen documentation for pioneer in html.

The `mainpage.dox` is just an placeholder so please feel free to rewrite it using markdown.

I also updated the `COMPILE.txt` file, by including a table of contents, section numbers, be consistent with section spacing, and added a new section on building documentation using natural docs or doxygen. The former changes were made since I always get lost in that file when using `more/less/most` from command line.

@eloquentmess might have input on how the documentation in `COMPILING.txt` for building on OSX is.

I think things like this can help newcommers get an overview of the code:

![2014-08-23-062255_1257x725_scrot](https://cloud.githubusercontent.com/assets/619390/4019690/ee777d8a-2a81-11e4-961f-75ed2ffdfebe.png)

![2014-08-23-062202_940x708_scrot](https://cloud.githubusercontent.com/assets/619390/4019692/f3acb306-2a81-11e4-82ec-4d65cd8324f8.png)