Allow HTTPI to be included in a Module/Class.

Alright, this one is a *bit* larger than my last pull request. :grin:

This allows HTTPI to be included in a Module/Class by using `extend self` instead of `class << self` within the HTTPI module definition.

I put all examples from `spec/httpi/httpi_spec.rb` into a shared example and made a class within `spec/support` that uses `include HTTPI`, then I described that class, as well `HTTPI`, as using those shared examples.

All specs are passing (besides the current pending one.)