Changed the package name from `main` to `saml`.

This will allow for gosaml to be used as a library as well as correctly fetched and imported with `go get`.

Additionally:

 * Removed the `main` function from authrequest.go (done in conjunction with the package name change).
 * Added additional "Installation" and "Usage" sections to the README.md to help get started.
 * Updated the copyright date.