added equipable gun which uses left click to fire at mouse cursor, added player (game entity) sprite rotation towards mouse cursor

- added equipable gun which uses left click to fire at mouse cursor
- player sprite rotation towards mouse cursor working (used new Vector
variables instead of Sean's variables from last commit)

sean, i made some changes to Seizonsha.cs. you can do a find for "//
ALEX" to see what i added

to-fix:
- collision after rotation is off (uses old hitbox as if player was
never rotated)
- mouse aim shooting is a tiny bit off sometimes
- alex variable names
- firing bullets randomly freezes player