More sections on devel guide

Getting started: sign up for Github, make a fork, etc
Setting up: install things, env variables
Building: compiling C ext and Cython
Testing: unit and doctests
Code style: PEP8 and the auto checker
Documentation: Building and serving the docs

All have links to reference material (most are software carpentry)
Link to github guides page

They some really nice tutorials.
Separate branching into a new section

Added some more tips for getting started: look at issues, tell what
you're working on, low-hanging fruit.
Mv docstring convs to Documentation

Reorder some things in the PR section as well.
Merge branch 'master' into devel-guide
Changelog entry for dev guide
Merge pull request #144 from fatiando/devel-guide

Completing the Developer Guide