SSL support

This adds a Reel::SSLServer class which can be used for terminating HTTPS.

A quick checklist of stuff I'd like to see before merging:

* [X] Reel::SSLServer
* [X] Server certificate API
* [ ] Support client certificates i.e. pass them through Reel::(SSL)Connection
* [ ] Support for *requiring* client certificates (i.e. refuse to process any connections without a client cert signed by a known CA)