Automated installer

Aaron;
Here's a first pass on an automated installer for gemini, including Python libraries, third party tools and data files. It's a one step command that automates all parts and leaves users with a working gemini executable in their path and ready to go. It installs python libraries into a virtualenv but linking out the gemini executable means that users never need to source the virtualenv or know that it exists.

This also allows easy updates of the code and data files, and the commit adds a `gemini update` command which grabs the latest release of the code and data files. From our side, we need to continue to update requirements.txt to point to released versions of gemini. Currently this points to git versions from the repo.

We could enhance this to pull in SnpEff as well (issue #101) since there is a CloudBioLinux build for that, but I wanted to get more eyes on the first version. Let me know if you have any questions or notice problems.