experiment with making revision identifiers untyped

The extant code assumes they are integers but this is

* probably not necessary
* unwittingly constraining in the face of storage systems
  that might like to use uuids, hashes, etc (c.f. twp.gitstore)

See #63

The changes here include:

* set default tiddler.revision on the model to None, not 0
* move invalid revision handling into the store from the handler,
  only the store can really know if a revision id is okay
* remove redudant revision handling in
  handler.tiddler:_delete_tiddler: if the tiddler can't be store.get,
  we're surely never going to be able to delete it
* adjust web.util:tiddler_etag to default to a '0' in new etags,
  as a convention. This preserves some edit conflict detection that
  is loose in the wild (only create this tiddler, never edit)

Would appreciate some review from @fnd (and anyone else who is
interested).

I will test against twp.sqlalchemy, twp.mysql and tiddlyspace to
get some better coverage, but given that all those use int revision
ids, some testing with gitstore anything else would be useful and
welcome.
Merge branch 'master' into generic-revisions
Merge branch 'generic-revisions' of github.com:tiddlyweb/tiddlyweb into generic-revisions