Added support for a level exit return stack.

A level exit can specify to push a return location onto the player's
return stack.  When the player goes through an exit that does not have a
destination level or entry, the return stack wil be popped and if
anything is present, the player will return to that location.
Fix tabs
Merge branch 'devel' into feature-returnstack

Conflicts:
	smc/src/objects/level_exit.cpp
Remove trailing space
No warning about level entry if not specified.
Implement saving and loading of return stack.
Clear player return stack when needed.
Fixed spaces to tabs on affected files.
Merge branch 'feature-returnstack' of git://github.com/brianvanderburg2/SMC into devel

Conflicts:
	smc/src/objects/level_exit.hpp