v2.0: Support DSA and ECDSA signing keys

[johnnyshields]

Fixes #661

Replaces #683

Depends on #711, please merge it first.

Currently RubySaml supports only RSA keys. The SAML standard can also support ECDSA and DSA keys. This PR adds support for both:

1. Validating IdP EC/DSA sigs AND
2. Using SP EC/DSA signing keys.

JRuby supports DSA but cannot support ECDSA due to this issue.

You may not use EC/DSA keys for encryption. An ArgumentError is raised if you attempt to do so.

It includes the following changes, which are all done in a backward compatible manner:

• When generating SP metadata/requests, settings.security[:signature_method] now ignores the "rsa" component of its user-set value and automatically uses whatever type of SP signing public key you actually set (e.g. a DSA key) plus the "sha" component of the value.
  • (Previously, only RSA was supported, so this doesn't break anything.)
• settings.security[:signature_method] supports shortcut values :sha1, :sha256, etc.
  • Shortcuts :rsa_sha256, :dsa_sha256 etc. also work, but as per above the "rsa"/"dsa" are ignored in favor of the SP public key type.
• Similar to above settings.security[:digest_method] supports shortcut values sha1, sha256, etc.
• New module XMLSecurity::Crypto is extracted from XMLSecurity::Document
• Cleaned-up code, including related to canonicalization

DONE:

• Ensure existing tests pass
• request_test.rb -- cover all 3 key types
• logoutrequest_test.rb -- cover all 3 key types
• slo_logoutresponse_test.rb -- cover all 3 key types
• metadata_test.rb -- cover all 3 key types
• Fix JRuby failures
• basic tests (settings, etc.) -- cover all 3 key types
• test EC/DSA keys when used by IdP
• disable encryption with EC/DSA keys

[johnnyshields]

@pitbulk this is ready for your initial review. I'll give it another pass tomorrow but the core changes and tests are basically working 🎉

[johnnyshields]

@pitbulk this is ready for your review and merge. (Please review #711 first)

[johnnyshields]

@pitbulk FYI I am now running this branch in prod without issue.

[pitbulk]

@johnnyshields, merged this one but rubocop needs to be fixed