Adding []/[]= support to Scope

The interface to scope is much clearer this way anyway,
but this is needed to integrate Puppet with Hiera[1].
It just provides hash-like behavior to Scope, which Hiera
and others can now easily rely on.

I also went through all of the code that used Scope#lookupvar
and Scope#setvar and changed it if possible, and at the same
time cleaned up a lot of tests that were unnecessarily stubbing
(and thus making it difficult to tell if I had actually broken
anything).

1 - https://github.com/ripienaar/hiera

Signed-off-by: Luke Kanies <luke@puppetlabs.com>
Reviewed-by: Nick Lewis <nick@puppetlabs.com>