various docs improvements
v0.26

 - Import FFI::Raw::Ptr in FFI::Raw to avoid a bug in perl < 5.10.1
   (GH#23) (thanks, @plicease!)
 - Require perl >= 5.10
 - Add boolean override for FFI::Raw objects (GH#28) (thanks, @calid!)
 - Fix build on non-threaded perl (like OpenBSD's) (GH#29) (thanks, @plicease!)
 - Use system libffi if available (GH#30) (GH#32) (thanks, @plicease!)
 - Allow resolving functions in the main program (GH#33) (thanks, @plicease!)