Refactored common functions to common.py.
Finds the shortest path between two actors weighted by billing position.
Added method to convert roman numeral to integer.
In shortest path search, ignore edges with undefined weight i.e. the billing position of the actor is undefined.
Fixed a bug in parse_data.py that resulted in duplicated movies.
parse_data.py parses actors.list in 10 minutes.  Previously took over 3 hours to complete.  I introduced a ParseBuffer which buffers data and writes to database in batch.
Revised graph to reflect the schema changes in parse_data.py.

Also fixed a bug in graph._backtrack: now it finds the optimal path.