allow custom ssl cipher lists

e.g.

c = Curl::Easy.new(url) do |curl|
   curl.timeout = 60
   curl.connect_timeout = 60
   curl.setopt(Curl::CURLOPT_SSL_CIPHER_LIST,"SSLv3")
end
Merge pull request #230 from bkr/set-ssl-ciphers

allow custom ssl cipher lists