feature(identifiers): Call functions with a helpful `this` argument

The `this` for function calls will be of the form:
 {
          $context: $context,
          $data: $data,
          globals: parser.globals || {},
          node: parser.node
}

Note that this deviates from what would be the ordinary Knockout
behaviour.

Closes #31
feature(operators): Change ==/!= operators to strict comparison (===/!==)

Fixes #25
fix(identifiers): Fix dereferences not setting `this` to members

Re. #31