[ADD] deduplication of website images being uploaded
  
There is a deduplication in ir.attachment, but it's only for FS-stored content
*and* it only deduplicates storage not models (as there are access rights
issues involved).
  
The goal here is to always return the same attachment when a user uploads the
exact same image multiple times (because it's simpler or whatever).
  
Initially tried to use a binary field & digest(), but search() blows up
because it tries to utf-8 encode raw binary data. So use char & hexdigest
instead.
  
_compute_checksum returns None if the provided attachment data does not look
like a website image attachment.
  
Unhandled: multiple existing matches, maybe a UNIQUE constraint on the
checksum field would be a good idea just in case.

cherrypicked from saas-3's xmo@openerp.com-20140303153855-5f2l8v0jq2mgb26f
which had to be backed out (as the patch adds a new stored field)