a couple of fixes and an enhancement

+ add convenience fn `connect-first` which connect to first connected sphero found. also changed README to recommend starting with this fn.
+ updated link to PDF docs (currently link 404s)
+ fixed sensor data hash-map keys. they had been keywords stringified with `str` instead of `name`, so the keys were strings like e.g. `":accelerometer-z-raw"` instead of keywords like `:accelerometer-z-raw`

Thanks for putting this together!