[WIP] v3.x - Replace REXML with Nokogiri

[johnnyshields]

Fixes #729
Fixes #707

Currently Ruby Saml uses a mix of REXML and Nokogiri. This is not ideal. We should use Nokogiri everywhere, because it can do everything REXML can do.

In addition, note:

• Ruby Saml is currently setting REXML::Security.entity_expansion_limit = 0 which affects it for all gems in the project.
• The above is not thread-safe, while Nokogiri is thread-safe by design.
• REXML does not use Semver, and exposes users to sudden breakage.

This refactor will probably get rid of RubySaml::XML::Document, etc.. Currently these are subclasses of REXML::Document. What we should do instead is make them procedural classes like DocumentSigner and SignedDocumentValidator. Hence I'm thinking it will be v3.0