Prevent loading bar from moving backwards

Following from pull request [#119](https://github.com/chieffancypants/angular-loading-bar/pull/119) - this solution seems to work nicely (and caters for the scenario you explained in the PR-119). 

Another thing to note is that I fixed the issue where the loading-bar gets removed from the DOM before it has the chance to move through to 100% width.

Lastly (and importantly) - I can't get the unit tests to pass with this change. The 'leave' method of animate returns nothing on line 234 which prevents the status to be set back to 0 (via the '_completeAnimation' function). Could you please assist in getting these tests to run!