Add cropping with gravity

[schorfES]

Instead of enabling cropping through the template tag {% ultimatethumb 'sourceset' image '320x400,400x500' crop=True %} by setting crop=True we need to optionally pass the gravitiy as string. See https://www.imagemagick.org/script/command-line-options.php#gravity for the possible values. The mapping could be the following:

• crop=True ---> gravity='Center'
• crop='C' ---> gravity='Center'
• crop='N' ---> gravity='North'
• crop='NW' ---> gravity='NorthWest'
• crop='NE' ---> gravity='NorthEast'
• crop='W' ---> gravity='West'
• crop='E' ---> gravity='East'
• crop='S' ---> gravity='South'
• crop='SW' ---> gravity='SouthWest'
• crop='SE' ---> gravity='SouthEast'

But I think we also need to support the imagemagick default values. So a full mapping can look like this:

• crop=True ---> gravity='Center'
• crop='C' ---> gravity='Center'
• crop='N' ---> gravity='North'
• crop='NW' ---> gravity='NorthWest'
• crop='NE' ---> gravity='NorthEast'
• crop='W' ---> gravity='West'
• crop='E' ---> gravity='East'
• crop='S' ---> gravity='South'
• crop='SW' ---> gravity='SouthWest'
• crop='SE' ---> gravity='SouthEast'
• crop='Center' ---> gravity='Center'
• crop='North' ---> gravity='North'
• crop='NorthWest' ---> gravity='NorthWest'
• crop='NorthEast' ---> gravity='NorthEast'
• crop='West' ---> gravity='West'
• crop='East' ---> gravity='East'
• crop='South' ---> gravity='South'
• crop='SouthWest' ---> gravity='SouthWest'
• crop='SouthEast' ---> gravity='SouthEast'

[schorfES]

Sorry, wrong project. Issue moved to https://github.com/moccu/django-ultimatethumb/issues/7