Merge pull request #2 from ricardojmendez/master

Implemented queue pausing
New IEnumerable to get the TickedQueue's items

Adding a way to iterate through the items, since I want to be able to
query them externally. Doing it as a IEnumerable to avoid exposing
direct access to the TickedQueueItem collection.
TickedQueue.Remove now has a success return value
BUGFIX: TickedQueue was ticking one item too many per update
New test for removing an item while it's enqueued
New TickedQueue.ExceptionHandler

Since TickedQueue ticks the items on its own, we can't really deal with
exceptions at the source unless we're notified.  Added a new exception
handler property.  TickedQueue will now catch exceptions and, if it has
a handler, notify it instead of throwing them.

Updated NUnit to 2.6.3
TickedQueue will not tick removed TickedQueueItems

Ran into an edge case with UnitySteer where a vehicle was being disabled
while the TickedQueue's work queue was being run through.  This had the
side effect that the item was removed from the main queue, but since it
was a "loop" item and already in the work queue, the Update method then
re-added it to the queue after ticking it.

Removing a TickedQueueItem from a queue will mark it as inactive. The
queue will now ignore inactive items.

Expanded tests.