Fix multi device support, Fix Mac OS X, Output JSON (#34)

* add param

* fix docopt

* make devices command output json

* fix log message

* add udid to all methods

* add udid to all methods

* fix activation output

* use correct format specifier for boolean

* fix activate command, fix log output

* refactor usb code

*  fix hanging bug

* remove global context variable

* refactor and clean

* refactor to use err

* improve help text

* remove unused vars