Uploader does not work in Safari (file.slice() method does not exist)

If you try to use the Ooyala uploader in Safari (latest stable version Version 6.0.2 (8536.26.17), for Mountain Lion), the uploader fails because the method "slice" does not exist on file objects. Apparently in Chrome, it supports slice directly per the W3 spec, but Safari is still dependent upon webkitSlice. This pull request switches the conditional from being a check for Mozilla or W3 standard to check W3 Standard, Mozilla, and then Webkit methods (in that order).