Performance Testing
-------------------

Browser objects keep up with how much time each request takes.  This can be
used to ensure a particular request's performance is within a tolerable range.
Be very careful using raw seconds, cross-machine differences can be huge,
pystones is usually a better choice.

    >>> browser.base = 'http://localhost:%s/' % TEST_PORT
    >>> browser.open('index.html')
    >>> browser.lastRequestSeconds < 10 # really big number for safety
    True
    >>> browser.lastRequestPystones < 100000 # really big number for safety
    True
