[#1304] Clear and keep current database during test runs

Add option '--reset-db' to drop and reinitialize database during test
runs
[#1304] change clean_db to rebuild_db in new_test/helpers.py

Otherwise new_tests will drop and recreate database
[#1304] update docs on 'reset-db' test option
[#1304] make sure database is rebuilt during travis test runs
Merge origin/master
Merge remote-tracking branch 'origin/master' into 1304-reset-db
Merge pull request #1304 from okfn/1304-reset-db

'--reset-db' option for when running tests