Add OpenSSL protocol plugin

Added new plugin which can be used to quickly test the supported SSL / TLS protocols.

```
$ sslyze.py --protocols www.google.com
   * Protocol Version:
       SSLv2                              Not Supported - TLS / Unexpected EOF
       SSLv3                              Supported
       TLSv1.0                            Supported
       TLSv1.1                            Supported
       TLSv1.2                            Supported
```
The plugin also supports usage of the newly introduced TLS_FALLBACK_SCSV service cipher suite value (e.g. to ensure it is properly supported on the server).

```
$ sslyze.py --protocols --fallback www.google.com
   * Protocol Version (using TLS_FALLBACK_SCSV):
       SSLv2                              Not Supported - TLS / Unexpected EOF
       SSLv3                              Not Supported - TLS / Alert inappropriate fallback
       TLSv1.0                            Not Supported - TLS / Alert inappropriate fallback
       TLSv1.1                            Not Supported - TLS / Alert inappropriate fallback
       TLSv1.2                            Supported
```

Requires updated nassl with https://github.com/nabla-c0d3/nassl/pull/19