Porting to Linux/Mac and other improvements.

+ Porting to Linux and Mac;
+ Proper computation of bounding boxes (extending it 20% beyond the computed dimensions, to account for cases where some parts of the animation frame were right at the boundary of the animation box and remained unclickable);
+ Support for objects appearing/disappearing in the middle of the mainline;
+ Support for Spriter projects made with the latest Spriter version (support for all Spriter versions from b5 upwards, to be precise);
+ Taking into account the png timestamps when deciding whether the project is up to date or not;
+ Basic command line options support (not really relevant for automated use by the autocompiler), adding the options "-f" and "--ignore-self-date". The first forces recompilation of the project, regardless of timestamp comparisons, and the latter ignores the timestamp of the scml compiler executable when doing the comparisons.