Commit by Kevin Walsh: Add quantification for auth.

This commit adds support for "forall" and "exists" statements in the
authorization language and AST. This is needed to set up clean ways of
describing the authorization of TPMs and OSes and programs in the standard
CloudProxy setup in datalog.

This commit changes some ACL and TPMTao code to port it to the new auth setup,
especially with respect to auth.Bytes and changes to some elements that used to
be exclusively auth.Prin but have been generalized to auth.Term.
Commit by Kevin Walsh: Add a Datalog-based Guard.

This commit adds an initial datalog-based guard that implements the Tao Guard
interface using the Go datalog engine from github.com/kevinawalsh/datalog and
the Tao auth framework in go/src/cloudproxy/tao/auth. It translates
authorizations into datalog statements and checks them using the datalaog
engine.

This commit adds basic integration of the DatalogGuard into the Domain.