Support use with multiple accounts

[mrfelton]

Issue Summary

Currently, everything is a static method on the SendGrid class. This means that it's not possible to use multiple instances of the class to connect to different SendGrid accounts (for example, since changing the API key will affect all subsequent code that uses the SendGrid class.

I'd suggest moving the API method to an instance method, and turn the existing constructor into a factory method that returns a new instance of the class with the configured API key. This way, calling sendgrid(API_KEY) will get you a new instance of the SendGrid class with the API key set for that instance alone.