Support for ca chain certificates

[CarbonCollins]

Hello,

Alexa would not connect to the server with the SSL cert that I had without also including the ca bundle (spent several hours of it working on my local machine and not on Alexa dev network to figure this one out)

I have forked and have added the code here and have made the ca cert optional as to keep the original functionality and to allow the inclusion of the cert if available.

[dblock]

@CarbonCollins You should PR your changes please, see https://github.com/alexa-js/alexa-app-server/blob/master/CONTRIBUTING.md

[tejashah88]

Resolved with #45. Closing...