Feature Dev

 * Switch to docopt (docopt.org)
 * Support data payloads for POST, PUT, etc.
 * Add `__elapsed__` to headers, to assert on timings
 * Support sessions (for cookies, keep-alive, etc)
 * Support `--vars=` option to pull in globals externally defined. (Needs review)
 * Support scripted callouts; e.g. invoke module which runs a `splinter` test.