Change to JSON key file breaks using relative paths

[jasonpolites]

This recent change to using a single JSON file for the key data makes sense but it now breaks if you specify a path relative to the root of the project. By default require(...) will search node_modules if the path specified is relative.

Recommend using more generalized file system operations (e.g. fs & path) so that applications can remain isolated from their environment

[rakyll]

I will be fixing it while I'm working on #46.