Well it looks like the signing stuff has been moved out to the SPFilter/etc. But the source is still a mystery.
if you use MAVEN, you can find needed 2.1.4 jar here https://repository.jboss.org/nexus/index.html#nexus-search;quick~picketlink
And If you don't use maven I have found that the project is no longer using svn for new versions, but they use github.
Anf I think that main code is here (there is a download link):
And main guide-post is here:
Hope it helps :-)