Template registration, first implementation

[clarle]

As discussed in #1021 and the linked gist.

Here's a first drop for template registration, including full tests, user guide documentation, and API docs.

@ericf and I were discussing a possible addition to the Y.Template.register API, where it would also accept a hash of template names to template functions. For example:

// Registers 'foo', 'bar', and 'baz' templates all at once
Y.Template.register({
     'foo': fooTemplate,
     'bar': barTemplate,
     'baz': bazTemplate
});

I started implementing something like this, but I wasn't sure what Y.Template.register(templateHash) would return. The current Y.Template.register(templateName, template) returns the revived template to maintain compatibility with the revive() API.

Pinging @caridy also for review.

[clarle]

I'm an idiot and PR'd into master again.