Optimize variable declaration
Lookup partials when undefined
Provide Handlebars.noConflict implementation

Allows for users who are loading via a global pattern to avoid conflicting with other instances of the library.

Fixes #887
Export the default object for handlebars/runtime

Maintains consistency with the prior export pattern, while avoids the knarly default loading issues.
Update to latest es6-module-packager
Make each helper data uniform

Provide @key and @last value for all forms of iteration.

Fixes #910