Fix a couple of memory and resource leaks.

[CruzR]

• Some local variables were allocated with new but never deleted. They now use std::auto_ptr to prevent that from happening again. std::unique_ptr would allow us to get rid of even more explicit deletes, but the style guide for supertuxkart forbids using C++11 features.
• Some irr::IReferenceCounted objects were not released with drop(). A new template class, AutoDrop, which works the same way as std::auto_ptr, takes care of them.

[hiker]

@leyyin since you are so good with coding style - do you want to have a look? Thanks!

[vampy]

Also, where does the style guide forbid the usage of C++11? EDIT: found the sentence, needs to be changed and going to have a discussion with @hiker about changing it.

[konstin]

afaik It was decided to use C++11 though it was never advertised because of problems with Mac OS X and older compilers.

[auriamg]

This was true until recently, though we have now moved to C++ 11 - the only restriction is that features should work in GCC 4.8 and VS 2013 (so no features that were implemented after that). I think unique_ptr was implemented relatively early on all compilers so it should be fine, though

[CruzR]

Alright, I've updated this commit to use std::unique_ptr instead.

[vampy]

Looks good, thanks!