Add support for HTTP Negotiate/SPNEGO authentication

[jheiss]

Only curb supports this, hopefully you're OK with functionality that doesn't work with all adapters.

[jheiss]

I should perhaps explain why I'd want functionality in httpi that is not portable to multiple adapters. I'm working with the jimson gem, a JSON-RPC library. It currently uses rest-client for HTTP client activity. It previously used patron, another libcurl wrapper. They switched from patron to rest-client for jruby compatibility. I'd like to use jimson, but need Negotiate authentication. So we have conflicting requirements:

• Some folks need jimson to work in jruby -> no libcurl wrappers
• I need jimson to support Negotiate -> need to use curb

So what I'm thinking is that I'll propose that jimson switch to httpi. The jruby users can have httpi do its default thing and use httpclient or net/http. I can tell httpi (through jimson) to only use curb and turn on Negotiate auth.

[rubiii]

to be released with v0.9.8. thanks!