Add filter for causal and non-causal gene to disease associations

[kshefchek]

Adds filter for causal and non-causal gene to disease associations and a few corresponding behave tests

Closes #274

Testing locally: http://127.0.0.1:5000/api/bioentity/disease/MONDO:0007947/genes?rows=10&association_type=causal http://127.0.0.1:5000/api/bioentity/disease/MONDO:0007947/genes?rows=10&association_type=non_causal http://127.0.0.1:5000/api/bioentity/disease/MONDO:0007947/genes?rows=10&association_type=both http://127.0.0.1:5000/api/bioentity/disease/MONDO:0007947/genes?rows=10

note that travis failed twice on tests/bioentity-function.feature:5 User fetches all GO functional assignments for a zebrafish gene