Automatic schema version updater for Cassandra CQL scripts

* Adds code that can loop through a list of CQL files and keep track of which files have been applied to a Cassandra cluster already.
* This will be used to keep the Cassandra schema updated when new column families are added and changed.
* Will create a column family of its own `schema_scripts_version`, that stores one row per script that has been executed.
Merge pull request #50 from iZettle/schema-version-updater

Automatic schema version updater for Cassandra CQL scripts