Improved /certificate/sign stability

Now the server shouldn't send (blatantly) invalid data to the signer *and* the signer should handle invalid data better.

* validate the inputs to /certificate/sign
* fixed a couple crashes when the signer was fed invalid data
* a signer crash doesn't crash the server
    * had to modify compute-cluster to return an error on the callback in case of a crash. PR https://github.com/lloyd/node-compute-cluster/pull/19

This also fixes #191