Added command-line ImageMagick implementation.

The command-line ImageMagick implementation can be enabled by setting
the image_library option to 2.
It might also be necessary to define the convert_bin and identify_bin
options if the convert and identify binaries are not found in the
system path.
Reject the processing if the errorThrown property has been set.

This allows aborting the processing queue via data.abort() call.
Use ProgressEvent instead of XMLHttpRequestUpload to check for XHR2 capabilities. Closes #2671
Catch imagick image detection exceptions.