Add revoke command

[namreg]

Resolve #43.
Any comments are appreciated.

[CLAassistant]

All committers have signed the CLA.

[mcpherrinm]

Thanks! I'll review soon. It looks fine from a high level overview

[mcpherrinm]

Sorry this has hung around for so long (the last month has been busy).

The only real concern I have with this PR is a lack of tests. I'm not too worried about that for now, but would appreciate at least a basic functional test.

note that we just merged another PR moving away from Godep and this will require your PR to be updated too.

[namreg]

@mcpherrinm Got it! I will add some tests asap and let you know when done.

[namreg]

@mcpherrinm I have done

[mcpherrinm]

One more nit: You've check in the certstrap binary. Please remove it from your PR (including from history) so as to not bloat this repo.

It would be nice if there was an option on the revoke command to revoke by serial (in case you don't have the cert file), but we can merge without that and add it in the future.

Otherwise I think we can merge.

[namreg]

@mcpherrinm I've removed certstrap binary, also rebased on master and squashed commits.

[mcpherrinm]

Great! Thank you for your contribution