Misc. small improvements and bugfixes

[ryepup]

A handful of minor stuff I found when my employer switched to Stripe:

• more convenient handling of null values
• bugfixes with explicitly passing :api-key
• keep up with some changes in alexandria
• export some of the error-handling symbols so I could handler-bind without using stripe:: symbols

And one major bug:

• close the drakma response stream! (we hit the linux system limit on open file handles due to a couple hundred TCP connections to Stripe hanging around.)