SerialManager: 2 sec sleep, autodetection skippable, lazy open (#10)

- configurable (default:2 sec) sleep after connect
- port autodetection can be skipped
- connect only if necessary (lazy open)
speed up tests (skip sleep)
exception instead of assert
renames: nr -> pin_number, name2int -> to_pin_number
shiftOut
support AD9850
new example: checkspeed
fix module name
optimize DefineClass
use NUM_DIGITAL_PINS constant
explicit use of SerialManager in examples
fix counter for 16MHz