Add encoding option to file transfer source

Hey - 

Azure blob storage uses shared access signatures for secure access of stored blobs.  Currently, the File Transfer plugin encodes the URI of the source before delegating to cordova for downloading.  The azure blob shared access signature is generated at the file level, and URI encoding it prior to using it as a source breaks the signature.  This pull request adds an option (encodeSource) to the download function of the File Transfer plugin, with a default value of true.  Explicitly setting this value as false prevents the source from being encoded.

Thanks.