Allow starting and stopping timed() manually

[tuukkamustonen]

Added support for manually starting and stopping a timer.

Now it can be used something like this:

@app.before_request
def before():
    request.page_load_timer = statsd.timed('metric')
    request.page_load_timer.start()

@app.after_request
def after():
    request.page_load_timer.stop()

[tuukkamustonen]

@yannmh Would you have time to review this and #154 ?

[yannmh]

Apologies for my late answer @tuukkamustonen 🙇 . I was busy working on a different project and missed your PR.

I'll review your changes next week and keep you updated. Thanks a lot for your contribution!

[yannmh]

Looks great. Thanks again @tuukkamustonen.

[tuukkamustonen]

@yannmh Thanks. Any chance for a new release?

[yannmh]

@tuukkamustonen yes, there are a few things I want to check/merge before.

I plan to release by the end of the week 🎉