Using c-ares-backed DNS resolution

[ntoshev]

I encountered some problems when trying to do many simultaneous http requests because the default DNS resolution in node.js uses getaddrinfo(3) syscall in a thread pool. I switched to c-ares backed dns resolution and this solves two problems:

1. it's faster
2. using DNS with high levels of parallelism is slowing down disk I/O too, see http://groups.google.com/group/nodejs/browse_thread/thread/6f564ac1dcd650bf

Hope you are willing to add it to the library because this is the proper way to handle it.

[ntoshev]

Also added support for timeouts and passing through agent parameter to http.request

[andris9]

Thanks!