Make --game_data path always include trailing slash

Closes #418

This also adds an error message for when the path doesn't exist.
Don't add slash to USER_PATH_DATA if there is alreay one
Fix post_power behavior for hazard powers
Merge pull request #429 from hean01/timeunit

Use parse_duration() for lifespan attribute of power def.
Merge pull request #419 from dorkster/bugfixes

Make --game_data path always include trailing slash