Support adding your own cert and key (like the Go cli)

[shazron]

I would like to add this support in a PR.

These options will be passed into the client.js constructor: https://github.com/apache/incubator-openwhisk-client-js/blob/c2de73b69db3c95fa874c009c5b9b92302eecbd8/lib/client.js#L84

... and these options will be set in the underlying needle module that will use them as cert and key options.

Any concerns?

[jthomas]

sounds fine - open the PR?

[shazron]

Thanks! See #152