base64 decode messages in POST bindings.

SAML 2.0 bindings spec #3.5.4 specifies messages should be base64
encoded before being URL encoded. Ruby handles the URL decoding
magically. This CL does the base64 decoding. Without this fix, the
POST binding implementation is non-compliant.
Merge pull request #5 from omb-awong/post-binding-baes64

base64 decode messages in POST bindings.