Add support for XML plists

[mheily]

As a simple short-term solution, we could use Python to internally convert between XML and JSON. Python comes with a build-in plistlib library to parse plists, and the built-in json library can emit JSON.

A longer term solution would be to write an XMLManifestParser class that uses libxml2 (or a C++ wrapper) to directly parse the XML.

The extension of the file determines what parser to use; .json is for JSON, .plist is for XML