Issue 61

Adding some fixes and improvements to @josenavas testing framework PR.  @josenavas, can you **pull upstream master on this branch** (needed to avoid duplicating commit history), review this PR (it is against the branch on your fork), and merge if you like?

Summary:
- Removed example and test config files in the base repo dir. We don't need this since the ag_config.txt file in the amgut dir acts as an example.
- Added support for an AG_CONFIG environment variable.  If it is not found, then the ag_config.txt in the amgut dir is used by default. If a filepath is set that does not exist, an error is raised
- The error type being raised on missing section was incorrect -- I fixed this in Qiita, as well (see biocore/qiita#307)
- In addition to erroring on missing sections/options, warn the user if they have supplied sections or options that are not expected (to warn them of the possibility that they made a typo)