bind context to this during execution

[rclark]

Fixes #35 by binding context to the function when it gets executed. This makes context functions and attributes accessible:

var streambot = require('streambot');

streambot(function(event, callback) {
  console.log(this.getRemainingTimeInMillis());
  console.log(this.invokedFunctionArn);
  callback();
});

cc @emilymcafee @zmully I know at some point you folks were looking for this.

[rclark]

Also, for the sake of logging, added a setTimeout that will log a message when the function has about 200ms of execution time remaining.