Fast lxml-based XML plist reader/editor/writer.

 * Arrays and dictionaries are lazily loaded.
 * Source data is internally maintained in an lxml document.


Parsing speed using my iTunes library:

    plist (http://pyug.at/Projekte/BinaryPlistLib):                 7.473s
    plistlib (http://docs.python.org/2/library/plistlib.html):      7.420s
    plistop (http://github.com/dw/plistop):                         0.428s


Serializing speed using my iTunes library:

    plist:          5.372s
    plistlib:       5.444s
    plistop:        0.253s
