Ideal v3 support

Just in time before August 1st. I have added full Ideal v3  support.

Notable changes:

Nokogiri instead of Builder
Moved XML building out of Gateway to its own Request class
All Ideal responses are validated

I'm currently using xmlsec1 to sign XML signatures. When xmldsig matures I will replace xmlsec1 with xmldsig. See this discussion why I'm not using xmldsig for the time being: https://github.com/benoist/xmldsig/issues/9

I haven't had time to update all the tests. (I will soon).