Allow custom AWS.SNS object in constructor

[SundeepK]

Should resolve #9, let me know if there's anything I'm missing.

[evanshortiss]

No worries, this does almost all we need. I'll merge and a add a property check.

What I mean by checking properties is just something like so:

var obj = {
  // Assume these are all defined...
  getName: getName,
  name: 'Evan'
}

// Check does it have the properties we need
if (obj.hasOwnProperty('getName') && obj.hasOwnProperty('name')) {
  // Do something
}

[SundeepK]

Wouldn't this require you to create a default SNS object and get the properties from that to compare whether the SNS object passed in has the same properties as the local version? Either way, I'll keep a lookout for your changes.