[WIP] v3.0 - Replace REXML with Nokogiri

[johnnyshields]

UPDATE: This PR is abandoned, but please leave it open. I will close it when Nokogiri migration work is completed.

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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

[pitbulk]

@johnnyshields, let's replace REXML with Nokogiri on V2 and in V3 in addition, we can let's get rid of RubySaml::XML::Document as you suggested.

[pitbulk]

@johnnyshields , any progress here? we need to drop REXML before the v2 release and I want to also plan to do that on master branch.

[johnnyshields]

@pitbulk it's way too big a change, and potentially could introduce issues. Let's aim to do it in 3.0 and release what we have as 2.0

[kwent]

Looking forward to this.

[johnnyshields]

Abandoned this branch, see my other PRs