Total execution time.

Hi,

This PR is about adding the total execution time in the profiler menu, like doctrine does it.
I also fixed the debug bar tooltip to use the same formatting as the profiler menu: when the total time exceed 1s, the result is displayed in seconds. Otherwise, the result is displayed in milliseconds.

![guzzle](https://f.cloud.github.com/assets/351471/567028/221b099e-c6a7-11e2-803b-ce2b5bb4bd05.png)