Add a difference method to SC.DateTime

This method allow to compute the number of weeks, days, hours, minutes,
or seconds between two dates.
Throw errors is the passed parameters are invalid

As suggest by dcporter an error is now throw for the methods: compare,
compareDate and difference if the parameters are incorrect.
The result should be rounded
Attempts to improve the documentation for %E formatter.  Removes mention of %e as a format parameter, because it is confusing.  Instead adds the full list of localized strings to the documentation (in case the person is working off the gem and doesn't have access to the SproutCore source) and also makes a note about %e's usage in the context of those localized strings.

Also slightly improves the documentation for `difference` method and cleans whitespace.