Introduce Poise stat

Closes #80

I went with a percentage chance here as opposed to a damage threshold
because I think it's easier to scale when balancing combat numbers. We can
always change the formula to a damage threshold later if that's the desired
method.

I decided to keep the hit sound effect when damage recoil is skipped, since
it's the only form of feedback when combat text is off.
Merge pull request #322 from dorkster/bugfixes

Introduce Poise stat
Fix enemy hit sound not playing

Closes #323
Merge pull request #324 from dorkster/bugfixes

Fix enemy hit sound not playing
graphics: optimized png graphics using matthiaskrgr/image_optim script.

Stats:
16 files changed in total
  16 binary files changed 14038 b (13 kb) -> 9762 b (9 kb) => [-4276 b (-4 kb)]
Merge pull request #329 from matthiaskrgr/optimized_graphics

graphics: optimized png graphics using matthiaskrgr/image_optim script.
Merge pull request #327 from stefanbeller/master

CMakeLists.txt: remove -pg on default.
add xgettext.py script since it was mentioned in readme.txt but not in the directory.
Merge pull request #330 from matthiaskrgr/master

    add xgettext.py script since it was mentioned in readme.txt but not in the directory.
Proofread and updates for consistency of Swedish translation.
Fix crash from freeing backgroundsurface twice in MapRenderer

`backgroundsurface` is already freed in MapRenderer::clearLayers(), which
is called in MapRenderer's deconstructor.
Update of xgettext.py to support extraction of strings, fixes
issue with extraction of string with , and " as it breaks in
specialcase handling of extraction logic.

Script is tested on flare-game alpha_demo which was a great
testcase which produced a broken pot file and failed msgmerge.
Merge pull request #334 from hean01/master

Update of xgettext.py to support extraction of strings,
Merge pull request #332 from dorkster/bugfixes

Fix crash from freeing backgroundsurface twice in MapRenderer
Merge pull request #331 from hean01/translation

Proofread and updates for consistency of Swedish translation.