Fixed issue #12 - private key str length variance.
Fixed issue #12 - private key str length variance.

The issue is that sometimes openssl appends "00:" to the key. The parsing
function is not expecting this, and parses it wrong. Fixed the issue by
adding a comment stipulating the string length must always be the same.

GoogleCloudPlatform/google-cloud-iot-arduino,GoogleCloudPlatform/google-cloud-iot-arduino,GoogleCloudPlatform/google-cloud-iot-arduino