fix lib.Network.encode_multipart_formdata in chrome 24+

BlobBuilder(named WebkitBlobBuilder) is deprecated in chrome 24+.
Use Blob constructor instead.

Old function using BlobBuilder is kept for compatiblity.

Links:
Deprecated of BlobBuilder: https://developer.mozilla.org/en-US/docs/DOM/Blob
Merge pull request #520 from dazzyd/bug-fix

fix lib.Network.encode_multipart_formdata in chrome 24+