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
Warrior powers Throw Knife and Throw Axe
Ranger powers Caltrops and Bear Trap
Caltrop and Bear Trap art sources
Lower Bear Trap requirement
Icons for Throw Knife and Axe
Use parse_duration() for lifespan attribute of power def.
Fix post_power behavior for hazard powers
Merge branch 'master' of https://github.com/clintbellanger/flare-engine
Bear trap now animated even when killing the target
Clean up some containers
Merge pull request #186 from dorkster/fixes

Clean up some containers
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
DevLab basic tutorial hallway
Remove duplicate warp zone map
Merge remote-tracking branch 'upstream/master'
Merge pull request #427 from hean01/master

 Remove alot of logic from the audio subsystem callback which