Fixed up wide image preview cropping

* Fixed a bug in image previews
* Added a cropMode
validator.isIn can only handle string values.

Added a validator.isBoolean function.
Outputting small, medium and large preview URLs if they are available.
Not generating a wide image if the source is too small.
Merge branch 'previews-fix' of git://github.com/simong/Hilary into previews-fix
Renamed skip variable to allowStretching