fix the command line problem

Now we can install this from github directly and use it in command line. (I verified in windows platform)

    $ pip install https://github.com/hay/xml2json/zipball/master
    $ xml2json --help

Originally it reports:

    $ xml2json
    Traceback (most recent call last):
      File "c:\Python27\Scripts\xml2json-script.py", line 9, in <module>
        load_entry_point('xml2json==0.1', 'console_scripts', 'xml2json')()
    TypeError: xml2json() takes at least 2 arguments (0 given)

Hope you can merge it ;-)