Apply prescaler and reset counter on timer start

[TheZoq2]

First of all, this commit fixes #57. Turns out that the prescaler value isn't applied until after an "update" event which gets triggered when the counter resets. This commit makes the update event trigger in the start function and then resets the uif flag which would otherwise indicate that the timer is already done.

This also fixes an issue where the following program would not wait for the second timer

loop {
    timer1.start(Hertz(2));
    block!(timer1.wait));
    timer2.start(Hertz(1));
    block(timer2.wait());
}

[japaric]

Thanks @TheZoq2

bors r+

(I feel like I have run into this and fixed it more than once before ...)

[bors[bot]]

Build succeeded

• continuous-integration/travis-ci/push