Add activity field to time logs

Thus, the name of the type of activity will not be as time_tracker_activity, will appear for selecting the values ​​that the administrator set up in Redmine.

This additional field is available on the screen after start time tracker.

![time_tracker](https://f.cloud.github.com/assets/519642/162929/8443743e-77de-11e2-98ba-669775d70914.jpg)