use libsecret-sharing's serializer to create generic sec-sharing crypto engine

In addition add a simple test case that tests that we can store and retrieve
data using secret-sharing (currently only ShamirPSS is supported). This does
not restart the bft servers -- I believe this will introduce "intersting"
problems as we have neither ordered-shutdown nor recovery
Merge pull request #10 from andreashappe/master

use libsecret-sharing's serializer to create generic sec-sharing crypto ...