Removed unused imports from database.py
Don't pull out genotype columns if they are not involved in the query.

Pull out genotypes only if necessary for the query (the query
involves the variants table and one of the query items has "gt"
to start).
Made genotype queries more specific.

Queries only grab the genotype columns if information is going to be
used from them.
Merge branch 'master' of github.com:roryk/gemini

Conflicts:
	gemini/gemini_annotate.py
Merge branch 'master' of github.com:arq5x/gemini
Fixed the len(iterator) gotcha.
Merge pull request #130 from roryk/master

Queries involving genotype information are more specifically flagged