working draft for core schema/spec

Here's another schema, based on discussions with Rich, and trying to 
further simplify.  Also, I started a document describing it.

Changes:
  - calling this a "working draft"
  - added <SignResponse> element
  - moved <SignedReferences> into <Options>, to simplify
  - replaced types with referenced elements in schema, i.e. type=SomeType 
-> ref=SomeElement, makes descriptions more concise and readable in the doc.
  - added descriptions of how processing occurs.  I think we have a good, 
simple, mini-core of functionality now, so that clients and servers can 
always interoperate on a minimum level, without using options.  Also, I 
think we have good, simple approaches for Enveloped and Enveloping signatures.
  - dropped a few of the less important options:
    - inserting signature into a document and returning that
    - returning transformed and/or untransformed documents
    - We could add these back, but I wanted to see if anyone cares

Trevor

http://www.oasis-open.org/apps/org/workgroup/dss/download.php/3715/oasis-dss-1.0-core-schema-wd-01.xsd

http://www.oasis-open.org/apps/org/workgroup/dss/download.php/3714/oasis-dss-1.0-core-spec-wd-01.pdf
http://www.oasis-open.org/apps/org/workgroup/dss/download.php/3713/oasis-dss-1.0-core-spec-wd-01.doc
(the word doc has colors, fwiw).