i2cbus: change the effect of empty read/write, change errors

i2cbus: fix typo in doc

i2cbus: remove the Wait method from the Conn interface

i2cbus: new MasterError type, add Addr and Master methods to Conn

bus/i2cbus: fix Addr.String, new MasterError type

new bus/i2cbus and adc/ads111x packages

first commit