A mostly empty version of the TPMTao that implements the Tao
Add support for TPM Tao implementation and Quote verification.

This commit adds implementations of the Tao interface over the TPM, using
github.com/google/go-tpm/tpm. It also uses that package to add support for
verifying TPM Quote values in attestations using the cloudproxy/tao/auth
framework.

This commit also adds some basic tests that use TPMTao as a Tao object to do
Seal, Unseal, Attest, and verification of a simple auth.Says.
Fixes for TPMTao along the lines suggested by kwalsh