experience points calculation
Ignore Xcode project/workspace user data
Fix Xcode build

* added OpenGL framework to libraries
* removed dubious header search path pointing to an absolute path
inside Xcode.app
MacPorts support for Xcode build

* added MacPorts library and include paths to search path
* updated build instructions in README to explain how to install
dependencies with MacPorts
Fix for issue #771 (Psi attack breaking targeting mode).
fix incorrect craft rearming
Compilers
Merge pull request #915 from sfalexrog/fixes

Fix for issue #771 (Psi attack breaking targeting mode).
center camera
Merge pull request #916 from redv/fix_craft_reloading

Fix: incorrect rearming of Craft weapons
Merge pull request #902 from hackbunny/xcode_building

Basic Xcode build improvements
Merge pull request #897 from redv/experience_calc

Perfectionism: experience points calculation