Completing the Developer Guide

Writing some more sections for the [Developer Guide](http://fatiando.readthedocs.org/en/latest/develop.html).

* Getting started: sign up for Github, make a fork, etc
* Setting up: install things, setup environment variables
* Building: compiling C extensions and Cython
* Testing: unit and doctests
* Code style: PEP8 and the auto checker
* Documentation: building and serving the docs

I tried to include some links to reference material for those new to software development. Most are links to [Software Carpentry](http://software-carpentry.org/).

There is a version of the docs of this PR on ReadTheDocs: http://fatiando.readthedocs.org/en/devel-guide/develop.html 

It will be updated automatically with the PR.

Fixes #25 (probably all of it)

## Checklist:

- [x] Code and docs have been spellchecked
- [x] Changelog entry
- [x] Include new dependencies in docs, requirements.txt, README
- [x] Documentation builds properly
- [x] All tests pass
- [x] Can be merged