Create an API for accessing the global object #249

[GabrielCastro]

I've implemented the api for getting a reference to the global object as per #249.
I've made it a method on scope such that the following works

pub fn call_console_log(call: Call) -> JsResult<JsObject> {
    let scope = call.scope;
    
    let global = scope.global();
    let console = global.get(scope, "console")?.check::<JsObject>()?;
    let log = console.get(scope, "log")?.check::<JsFunction>()?;
    let args: Vec<Handle<JsString>> = vec![JsString::new(scope, "hello world").unwrap()];
    log.call(scope, console, args.into_iter())?;
...

[dherman]

@GabrielCastro Beautiful work! :D

[dherman]

It looks like the Travis failure was a transient npm issue, and our appveyor config is busted right now, so I'm going to merge. (This is definitely not an acceptable state of affairs for the CI--we should get this fixed soon.)