Added ability to retrieve scope using escope.

Added the `getScope` function to `eslint.js`. In the context of a rule, calling `getScope` will return an object representing the current scope.
Created no-label-var rule.

The no-label-var rule disallows creating a label that shares an identifier with a variable.

See: http://jslinterrors.com/a-is-a-statement-label/

Fixes #29