Support path OR string
I'm re-opening this PR that @astitt-ripple started. I cleaned up index.js a bit to follow the node conventions.
I took the liberty of polyfilling Promise since it's clear from our .travis.yml that we support Node 0.10.
I also took the liberty of returning the parsed certificate in the verify callback, since we already have that info and it will likely save people some hassle depending on their use case.
This PR is an updated version of https://github.com/Southern/node-x509/pull/47
@Southern You get a chance to look at this?
Heads up that I believe there are some build issues that need resolving on this PR. It usually works, but I have experienced some flakiness when building it as a dependency of a dependency.