run tests for sqlite, mysql, posgresql databases

mysql supports milliseconds in datetime fields since version 5.6. For passing all tests we should add 'id'  field in ordering fields to determine correct order, add time.sleep(1) statements in some tests.
fixed testing settings and travis config
fixed database name
improved transaction management for read trackers
Merge branch 'master' into permissions_mixin

Conflicts:
	pybb/views.py