Replace the call to the system plutil for a python library

[kronenthaler]

Currently this project has a hard dependency on a system command (plutil) it'd be nice to have the ability to switch it a python library.
Reason 1: quick patches could be written in a non-mac computer.
Reason 2: travis automation could be not running on a mac-environment or it could not have access to the system commands.

[kronenthaler]

Biplist: doesn't read the new format
plistlib: only parses xml plist -> depends on plutil to convert into xml first (current approach)

Alternatives: (http://www.monobjc.net/xcode-project-file-format.html)

• create a parser class
• embedded into the manipulation of the project => all sections have to be defined as classes