Pin the Mozilla cert hash.

[Lukasa]

This adds support for pinning the Mozilla leaf certificate.

Some caution needs to be taken here, because this can change underneath our feet. I may need to write some monitoring software to live on my local machine that will check whether Mozilla's leaf cert has changed so that I can proactively respond.

[Lukasa]

Resolves #4.

[sigmavirus24]

:+1: LGTM