Version 0.1.07

Backend changes:
* Changed the way enemy hitpoints are updated--math is done by php rather than database.
* Enemies now die when they are at or below 0 hitpoints. Fixes #50
* Enemies drop their 'corpse_item' on the tile when they die.
* Dead enemies have their reference removed on `character_map_tile` when they die.
* Dead enemies are removed from the `enemy_map` database when slain.