[WIP - NOT READY] Support DSA and ECDSA signing keys

[johnnyshields]

Fixes #661

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.

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

Things this PR does NOT do:

• I haven't yet looked at encryption. Probably, DSA/EC keys won't work with encryption (I'm unsure whether they actually should work, given that DSA is supposedly for signing-only.) To use the functionality in this PR, you must either:
  • Disable SP encryption when using EC/DSA keys OR
  • Use separate SP signing vs. encryption keys (supported in Add sp_cert_multi to facilitate SP cert/key rotation #673), and make sure the encryption key is an RSA key.

TODO:

• Ensure existing tests pass
• Test SP EC keys on metadata
• Test SP EC keys on messages
• Test SP DSA keys on metadata
• Test SP DSA keys on messages
• Test IdP EC keys on metadata
• Test IdP EC keys on messages
• Test IdP DSA keys on metadata
• Test IdP DSA keys on messages
• Investigate encryption

[johnnyshields]

I'll rebase this on v2.0 branch once other PRs are merged.