basic Nm template engine

This engine renders the given path, in the source path, using Nm.
Sanford then takes the rendered template data and serializes it to
BSON as needed.

This engine memoizes the Nm source and binds to an overridable
scope name method, 'view'.

This should be all that is needed to have a working sanford Nm
template engine.

Related to redding/sanford#85.

@jcredding ready for review.
@kellyredding - You have a closing parenthesis but don't have an opening one for the `template_source` method.