v2.x - Nokogiri Refactor Part 5: Extract out RubySaml::XML::Decryptor + Decoder

[johnnyshields]

Includes changes from #745, please merge that one first.

This does the following changes:

• Extracts out Base64 decoding/deflate to RubySaml::XML::Decoder
• Extracts out XML decryption logic to RubySaml::XML::Decryptor
• Centralizes namespace constants in RubySaml::XML module
• Uses Base64.strict_encode64 everywhere. The only difference between Base64.strict_encode64 and Base64.encode64 is that the latter doesn't insert newline chars, while the former does. By using strict_encode64 we can remove a lot of .gsub("\n", "") calls.
• Performance: Memoize the SAML 2.0 Protocol object (currently re-loading the file each time we validate)