Add AuthnRequest signing

In conjunction with the [README.md additions](https://github.com/mattbaird/gosaml/pull/4/files#diff-04c6e90faac2675aa89e2176d2eec7d8R70), I've added a wiki page with basic cert generation steps.

As noted in https://github.com/mattbaird/gosaml/pull/3, the github diff for this PR is a little convoluted because of the renaming of `authrequest.go` to `authnrequest.go`.