Added a new method to generate signed requests.
Added AuthnRequest signing via xmlsec1.

 * Separated `struct` definitions into an authnrequestInterface.go file (using a C convention) to make authnrequest.go more readable.
 * Updated LoadCertificate in certificate.go to read, parse, and prepare public keys from X.509 certificates for inclusion in AuthnRequests.
Cleaned up struct names, added signed example to README.md.
Fixed the encoding and parametrization of the request URL.
Merge pull request #5 from amussey/add-signing

Add AuthnRequest signing