Check for parse_ssl_certificate method before use

[jrchamp]

Avoid immediate exit when the parse_ssl_certificate method is not defined.

[dmiller-nmap]

I agree that we should gracefully handle the case where openssl is not included. But I don't think that we can meaningfully report a handshake score without parsing the certificate. I'd accept a patch that does either of these:

1. detects the missing function at the beginning of execution and does not display or calculate a score, but simply reports the ciphersuites discovered, or
2. replaces the OpenSSL X509 parser with one written in Lua.

[jkryanchou]

is any pull request send at Github would be merged to svn official repository ?

[p-l-]

@ryanchou1991 see #127.

[jkryanchou]

@p-l- I got it, Thanks :) I will make my contribution to this project all my best effort: )