Support for standalone operation

Specify standalone=True to the I3statushandler constructor
to run i3pystatus without i3status in the background.
i3pystatus won't read input from stdin or any other
file object specified with input_stream.

The keyword argument interval specifies how often output should
be generated. The default is 1 (second).


Sorry guys for changing the way i3pystatus "way of operation"
is set so often. If you're want the "self-contained" mode
(you execute i3pystatus, i3pystatus automatically starts
i3status), don't set the file attribute, but pass the file
descriptor of the pipe as input_stream like this:

process = subprocess.Popen(["i3status", "-c", "~/.i3/status"], stdout=subprocess.PIPE, universal_newlines=True)
status = i3pystatus(input_stream=process.stdout)

On a side note:
The main class name has been changed to i3pystatus, but I3statusHandler
is still available as an alias. Use whichever you prefer :-)
(Linux is about choice after all)