Short circuit evaluation

[RoboPhred]

Allow BinaryExpression to handle short-circuit evaluation. This is needed for many use cases involving potentially-null values.

Example:

    var variables = {
        value: null
    };
    var src = 'value && value.length';
    var ast = parse(src).body[0].expression;
    var res = evaluate(ast, variables);

In javascript, the value will be a falsey, so js does not try to evaluate value.length. However, currently BinaryExpression tries to evaluate both sides of the expression before performing the operation. This leads to a crash, as length cannot be retrieved from a null value.
The result is the common javascript idiom foo && access(foo) will not work in static-eval

The fix is to evaluate the left side first, and only evaluate the right side if the left side is not falsey.

[goto-bus-stop]

Thanks! we should do the same for ||. would you mind adding that to this PR too?

[RoboPhred]

Implemented short circuit evaluation for OR.

[RoboPhred]

Reworked commits to fix github username / email.

[goto-bus-stop]

Thanks, LGTM!