Show pause status at /time_trackers index action

[otaviofcs]

At action index on time_trackers controller we should be abble to see if the time trackers are paused or not. A simple status watch. One more column in both view's table (Your time tracker and Other user's time trackers). If you like it, I can fork the redmine_time_tracker and patch it. I already did it for my personal use.

[delaitre]

A 'Status' column just before the 'Action' one seems to be a good idea.
Moreover, it will be needed when several (per-user) time trackers will be allowed.

Sending a pull request will be appreciated. Thanks!