Improve SSL support and implement SPDY

Hi,

This PR implements 2-4 in #409 and resolves #429. Change list is:

1. Use `listen 443 ssl` instead of `ssl on`
2. Common SSL settings (ciphers, protocols, etc) are not duplicated per vhost and are now in the `dokku.conf` file
3. Added `ssl_session_cache` and `ssl_session_timeout` directives as outlined [here](http://nginx.org/en/docs/http/configuring_https_servers.html#optimization)
4. Moved wildcard SSL declaration to the `dokku.conf` file to reduce memory footprint
5. Allow symlinks for the `server.crt` and `server.key` files instead of forcing users to copy the actual files. This also allows the certs to be stored in the standard OS directories while maintaining compatibility.
6. Updated the `README` appropriately
7. Implemented spdy support (`spdy` directive and the `Alternate-Protocol` header)
8. Change certificate folder name from `ssl` to `tls`
9. Add note to `README` about the HSTS header

Thanks!