Added a testing section to the developer guide.
Merge pull request #42 from mcflugen/add-testing-docs

Added a testing section to the developer guide.
Fixed issue #33.

Fixed
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* Updated the installation instructions to only list the recommended
  ways of installing landlab. For Windows and Mac this is from GitHub
  with the GitHub GUI, for Linux it's from the command line with git.
Merge branch 'fix-issue-33' of github.com:mcflugen/landlab into fix-issue-33