Strict mode support

fixes #129 and #130 
I don't think it's correct to set the `checkForStrictMode` flag here. See http://www.ecma-international.org/ecma-262/5.1/#sec-14.1 -- the directive prologue consists of *all* the leading string literal expression statements, of which any might be "use strict"

e.g. this code snippet *is* strict mode and should error:
```ts
function foo() {
     "extra awesome";
     "use strict";
    var static = 0;
}
```