[#164] change activity parsing commit/flush behaviour

instead of commiting every 50 activites, flush all activites to the
database. If more than 1000 sqlalchemy objects have been flushed, then
commit to the database.

Change flask-sqlalchemy debug mode to off
[#164] commit at end of parse_resource
Merge pull request #164 from IATI/164-memory-management

parse_resource speed ups and memory management