Improve detection of operational system and remove unused/obsolete fields.

Remove Globals.ON_MAC and Globals.ON_WIN. We now rely on IroniaCorp-Commons Computer to detect the current operating system.

Remove obsolete configuration fields (ps, pdf, and useNativeFilechooserDialog).
Merge branch 'master' of github.com:magsilva/jabref

Conflicts:
	src/net/sf/jabref/bst/BstLexer.java
	src/net/sf/jabref/bst/BstParser.java