Add MiniMagick.validate_on_create and MiniMagick.validate_on_write flags

Disabling these flags can save lots of CPU time especially when used in `carrierwave` uploaders with multiple processings
Merge pull request #205 from andreychernih/optional-validation

Add MiniMagick.validate_on_create and MiniMagick.validate_on_write flags
Fixed extracting layers from image

Before it threw an exception because #{filename}.#{newformat} didn't
exist, but when extracting layers we should check for #{filename}-0.#{newformat}.