Sass custom function module hacks

Ensures all of sprockets old sass custom functions are only included while it invokes the engine.

Also exposes a public api for people to add new custom functions that are scoped to the engine. People should avoid hacking with `Sprockets::SassFunctions` and do something like:

``` ruby
Sprockets::SassTemplate.new do
  def my_sass_fun; end
end

# or

module MyFunctions
  def my_sass_fun; end
end

Sprockets::SassTemplate.new functions: MyFunctions
```

Closes #594.

Still needs some cleanup.

/cc @rafaelfranca