TDD unit tests for template registration
Initial implementation of template registration
User guide documentation for template registration
Remove unnecessary type-checking in template registration
Rename _cache to _registry in template registration
Make `Y.Template.render()` _always_ return a string

This allows YUI configurations which handle errors and allow the program
to continue running to always receive a string return value from the
static `render()` method.
Tweak user guide and API docs for Template registry feature
Update Template's HISTORY
Improve template registry example by using precompiled templates
Small typo fix in documentation
Additional API docs to template-base
Fix small mistake in engine revival
Add URL to HISTORY.md update
Merge branch 'template-registration' into dev-3.x