ORA-37 Explicit action to begin turbo mode.

This is a small change to modify the way Continued Grading is initiated. 
See https://openedx.atlassian.net/browse/ORA-37

Initially, continued grading occurred when you expanded the peer assessment section after you had already completed your required grading. This became an issue as much sections of the page allowed collapse / expand. 

This change is to make continued grading more explicit by introducing a button. See the attached screen shot.

![image](https://cloud.githubusercontent.com/assets/235752/3457842/245fbfe2-01fc-11e4-89ba-0c5ae006099a.png)

@wedaly I did not add any tests; think there's something worthwhile in testing the change of initialization events in the javascript?
@marcotuts Could you take a look at the screen shot and see if this is alright from a UX perspective?
@srpearce The text on the button is new, just wanted to make sure this was logical. 
Now that you've removed the "out of" text, could we translate only "Complete" instead of including the markup?