fix whitespace and ruby warnings
Pass template extensions through to partials

- move the new TemplateContext code into mixin/template and consolidate
  with ChefContext
- Copy extension modules from parent template to partial template.
- Functional tests for helpers with partials.
Move helper module compilation inside template resource

- Resource::Template compiles helper methods/module bodies into a
  collection of modules for inclusion into the template context.
- Resource::Template checks for obvious invalid input to helper
  definition methods.
Add inline doc for TemplateContext