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.


@mattbaird: I'm not sure about your use case for this library, so if this change is stepping on toes, let me know and I can keep it in my own fork.  My goal of this was to allow for improved organization while going forward and adding new features (ie, AuthnRequest signing is still in the works).