Merge partials and helpers options with globals

Fixes #163 

@wycats Wanted to highlight this change as it impacts the namespace "api" on the compiler, moving that functionality to the options section. I don't know if that functionality is commonly used but this is a breaking change for anyone who is doing so.

This also simplifies a few things in the preamble, moving them into the `Handlebars.VM.template` method.