- `Mixpanel.engage` function added to update user profile
 - `Mixpanel.track` and `Mixpanel.engage` will both support a list of maps
 - When given a list, these functions expect the maps to conform to the structure defined by
    the mixpanel HTTP api documentation (linked from README)

 - Mixpanel.Client now only accepts lists of events
 - Mixpanel facade normalizes single events to lists of maps

 - Note that these don't actually contain any asserts
 - And the ExVCR integration isn't working
 -- AFAICT the HTTPC adapter doesn't work with a custom :inets profile
 -- Even when using the default profile, the Mixpanel API is async, so need to wait in tests before asserting somehow