Make SSL/TLS version configurable

It seems that some servers and client operating systems don't work well together when negotiating the SSL/TLS version. In particular, using Ubuntu 12.04 against an IBM WebSphere Application Server 6.1 doesn't work when the endpoint is an `https://` URL.

The work-around is to force the client to use TLS 1.0. Unfortunately this is not currently possible in Three.

See the question [Python HTTPS requests (urllib2) to some sites fail on Ubuntu 12.04 without proxy](http://askubuntu.com/questions/116020/) on StackOverflow and Lukasa's blog post [Choosing The SSL Version In Python Requests](http://lukasa.co.uk/2013/01/Choosing_SSL_Version_In_Requests/) for more information.