Added localization and more flexibility regarding NSDateFormatters

I replaced all hardcoded strings with NSLocalizedStringFromTable and added English and Dutch MHPrettyDate.strings files.

I also added 2 extra public methods to allow people to provide their preferred NSDateFormatter for dates and times that are not stringified (instead of the hardcoded MM/dd/yyyy).

I also updated the .podspec to register the *.lproj files as resources.