Corrected some English phrases

Corrected some English phrases (up to rest.inc.php)
Well, there are arguments for and against it, see http://en.wikipedia.org/wiki/Email#Spelling. But at least the same spelling should be used everywhere. I found both in MODX.

There are quite some inconsistencies in the English language files (I already talked to John Peca about this). They should be eliminated some time.