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
Remove alot of logic from the audio subsystem callback which
removes the problem with audio threads modifying containers while
game thread iterates over it.

Fixes issue #425
Use parse_duration() for lifespan attribute of power def.
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
Merge remote-tracking branch 'upstream/master'
Merge pull request #427 from hean01/master

 Remove alot of logic from the audio subsystem callback which
Merge branch 'master' of https://github.com/clintbellanger/flare-engine