Merge pull request #2 from amussey/changed-package-name-to-saml

Changed the package name from `main` to `saml`.
Renamed authrequest.go to match the SAML XML name.
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.