Add test to provide sanity check for Prmd#render

Use the core parts of test/commands/verify_test.rb to define a basic
sanity checking test for Prmd#render. For now, the test essentially
verifies that Prmd#render successfully produced some Markdown. This will
at least let us know that it didn't throw an exception. Over time, this
test can grow to assert more specific results of Prmd#render.
Fix interagent/prmd#160

- Add test to reproduce the bug described in interagent/prmd#160
- Fix the bug:
  - In lib/prmd/templates/schemata/helper.erb:
    - Remove the sorting that attempted to always render the "id"
      property as the first property in the docs [1]
    - When processing "anyOf", gracefully handle a field that doesn't
      have a "description" attribute and/or an "example" attribute
  - In lib/prmd/schema.rb, gracefully handle an "anyOf" that doesn't
    have a "$ref"

[1]: For more details, see recommendation in
https://github.com/interagent/prmd/issues/160#issuecomment-54753071
Merge pull request #161 from jasonrudolph/fix-render-bug-for-any-of

Fix error generating docs when "anyOf" defines schemas for a property