Preliminary git support

@danmaclean This PR adds a series of new classes that export the database to the folder structure that we've discussed. 

At the moment, it creates the folders for all organisms, genomes and experiments and writes out YAML files for each of those as well. 

The YAML files are basic and don't include the created_by information that we've discussed. I'll be adding that in next alongside the the GFF and FASTA output.

To create a repo you use the rake task with `rake repo:export`.

Now that this is in place, it raises a whole host of questions regarding how this directory structure will be put into Git/GitHub. I'll outline those in #27.