Added --url option to iptest

Allows `iptest js` to be used against remote servers.

Example usage: `iptest js --url='http://localhost:8888'`

Pinging @rgbkrk who requested this.
`unicode` is a NameError on Python 3. You can use [py3compat.unicode_type](http://ipython.org/ipython-doc/dev/development/pycompat.html#IPython.utils.py3compat.unicode_type) if necessary.