More flexible blobstore support

 - Pass the input name and the filename in the getBlobstorePath call
 - The server can return list of extra parameters to send when uploading
   the file. Ex. <blobparam name="acl">public-read</blobparam>
 - The server can a return a name for the input field

Having these changes it is possible to use GwtUpload to upload blobs
directly to an amazon S3 bucket.