Allow non-global Stripe key configuration

[jkassemi]

I'd like the possibility of using this library in environments with Client objects outside of the default set, which this change allows.

client := &PlanClient{ Client{key: "non-default"} }
client.List() // Use 'non-default' as key.

[jkassemi]

And because I worked a little later than I should - added feature: account endpoint!

I know this makes this pull request undeniable, but if you do want to see these features separated I can do that ;)

[bradrydzewski]

awesome! I've been meaning to remove the global key, so this is definitely a welcome change. I'm definitely going to merge this but first I need to run a bunch of regression tests against drone.io just to be safe. Bear with me!