Cleaning queue if clearCroutonsForActivity has not been called

[jblernout]

Manager.java line 210 :
if (null == activity || activity.isFinishing()) {
return;
}

Instead of just returning is isFinishing is true, I think clearCroutonsForActivity should be called with activity in parameter.

If the developer forgot to call clearCroutonsForActivity when destroying its activity, the library won't display croutons anymore event after switching to another activity and the queue will keep growing.

You could also throw an exception to let the developer know he did something wrong.

[keyboardsurfer]

Relevant for #24