clean up XML::Schema

[flavorjones]

What problem is this PR intended to solve?

This branch is a WIP addressing several issues with Schema.

On branch:

• improved documentation for XML::Schema and XML::RelaxNG
• Nokogiri::XML::Schema#validate behaves differently in MRI and JRuby #1282

Planned for branch but not yet there:

• make error handling consistent when the XML is malformed or empty:

  • XML::Schema#validate(filename) returns 0 errors for malformed documents #642
  • Empty string does not fail schema validation with JRuby like it does with MRI #783

• pick one: Schema.new parsing a doc, or Schema.read_memory:

  • [debt] make XML::Schema.read_memory an alias for XML::Schema.new #2113
  • [bug] [jruby] RelaxNG.read_memory does not work #2115

• look into supporting xmlSchemaValidateStream to avoid unnecessary re-parsing

  • Extend nokogiri API to support libxml API call xmlSchemaValidateStream #222

Have you included adequate test coverage?

... in progress ...

Does this change affect the behavior of either the C or the Java implementations?

• Makes the two implementations consistent in how errors are handled by #validate and #valid?
• ... in progress ...

[flavorjones]

Rebased.