Start working on v3.7

[rowanwins]

• [x] add support for base64 images. resolves #543 and resolves #474
• [x] add the accepted flag to manually added files. resolves #529
• [x] Add support for uppercase filename previews
• [x] remove use of deprecated lastModifiedDate. resolves #482
• [x] make manual thumbnail fit consistent but overridable via CSS. resolves #455
• [x] add new prop for Dropzone.confirm

Entirely overhaul S3 approach

• [x] Ensure standard dropzone events are emitted, resolves #506
• [x] Allows cancelling of uploads. resolves #538 , resolves #514

[fnagel]

Any chance to include the new "displayExistingFile" method? See #560