* connection.c : implement connection_ping and enhance #connected?

This commit adds a "ping" method to the Connection class and uses
it in the #connected? method.

How to test this:

1. redis-server  --timeout 1&
2. cat >> test.rb << EOF

  conn = Hiredis::Connection.new
  conn.connect("127.0.0.1", 6379)

  puts "Connected? : " + conn.connected?

  sleep 2

  puts "Connected? : " + conn.connected?

3. rake compile && RUBYLIB=lib ruby -rhiredis test.rb
4. The last call to #connected? will return false because the underlying
   method call to #ping will raise EOFError.