SFML 2 upgrade

[mxnemu]

short: I would like to upgrade master to SFML2.

Why I didn't merge, yet:

I already upgraded Last Escape to SFML2 in a separate branch. I didn't merge it with the master branch, yet, because SFML2 is currently officially not marked as stable and there are no precompiled binaries. So I think that some of our contributors could may have issues compiling it.

Why I would like to merge now:

I'm currently adding new features in my own branch that is based on the SFML2 branch. I would still like to merge it with the master branch, because there are new features and there are some bug fixes that are partly based on changes in SFML2 (some timing things and positioning of the teleporter-coordinates). And as I continue developing Last Escape it would suck to write everything twice with the timing and coordinates adjusted for each SFML version.

Also there are not many upcoming changes for SFML2 : https://github.com/LaurentGomila/SFML/issues?milestone=5&state=open and I don't think that any of these will disable the code that we are going to write for Last Escape.

Please tell me if you would like to upgrade now, or wait until it's officially stable and precompiled binaries are published.