Allow SpreadElements to appear anywhere in arrays and function calls.

Per https://code.google.com/p/esprima/issues/detail?id=378#c3 and https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_operator.

This is blocking writing proper tests for [es6-spread-rest](https://github.com/bmeck/es6-spread-rest), a project to convert spread/rest to ES5.