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.
The new tags are optional so the Uploader is fully backward-compatible.
S3UploadServlet sample
merging with pull request #2