Merge pull request #2 from Youka/master

Several improvements in design, safety and performance
added libass warning&error catching and storing in ASSI_State
libASS is wrong, the library's name is libass
a copy of the header is stored in the state now
removed useless sizeof

Character size is always 1 byte and will (probably) never change. Additionally the copy source/destination must have the same type but was ignored so far.
improved user safety

Programs shouldn't crash when users forget to check for successfull initializations.
Multiple events initialization shouldn't cause memory leaks.