create new cursor if cursor.count() fails
fixed oplogmanager exception handling and tests

Conflicts:
	mongo_connector/oplog_manager.py
	tests/test_oplog_manager_sharded.py
Ignore oplog entries that don't have a namespace

Conflicts:
	mongo_connector/oplog_manager.py
ignore error if index already exists
update CHANGELOG.rst

Conflicts:
	CHANGELOG.rst