enable pkg-config on osx

addresses my latest comment on issue #14
Merge pull request #26 from zach-binary/patch-1

enable pkg-config on osx
- Added itchio.pri to store persistent build configurations that would
  otherwise be lost if itchio.pro were to be automatically regenerated.
- Added C++11 support and a few compile-time error checks.
- Improved the AppSettings interface and renamed the settings file from
  'settings.scratch' to 'itchio.ini'.