Metrics bugfix and .decorate alias

Creates a .decorate alias for the iopipe wrapper, such that:

// import iopipe as usual
module.exports = iopipe.decorate(function handler(event, context) { …

Also fixes a bug in custom metrics causing metrics to carry across invocations.

v0.1.1 - metrics, fixes Serverless Framework local exec

• add custom metrics in your functions!
• fixes a bug with local execution when using Serverless framework