Fix scaling images on iOS which are not subsampled, but do suffer a v…

…ertical squash.

Added sourceWidth, sourceHeight, top, right, bottom and left options …

…to allow arbitrary cropping. Closes blueimp#31

Moved orientation functionality into a separate file.
Added the Jcrop plugin to the demo to visualize the new cropping
features.

Added contain and cover options. Closes blueimp#30.

Updated path to canvas.toBlob polyfill library.

Added missing tags for Exif version 2.3. Fixes blueimp#25.

Added comment for the maxMetaDataSize option.

Provide the loadImage.blobSlice method only if the browser supports s…

…licing Blobs.