Improved systemtools.Timer.

You can now access the Timer from within the `with` block, like so:

    with systemtools.Timer() as timer:
        for _ in range(5):
            for _ in xrange(1000000):
                x = 1 + 1
            print timer.elapsed_time

All context managers can be accessed from within their `with` block so long as
they return a reference to themself from their `__enter__()` method.
Merged master.