Add staticmethod decorators for methods in TMDB class

Without the staticmethod decorators you would get e.g. TypeError: unbound method _request() must be called with TMDB instance as first argument (got str instance instead) and a similar TypeError for _set_attrs_to_values. This was also discussed in StackOverflow thread: 
http://stackoverflow.com/questions/20550738/typeerror-unbound-method-method-name-must-be-called-with-class-name-instanc.