Implement missing commands

[newpavlov]

Currently following commands are not implemented:

• FORCE_SCAN 0x21: this command is analogous to the SCAN command, but also returns debug information. Not fully documented.
• EXPRESS_SCAN 0x82: will run scanner at the sampling rate as high as it can be. For A2 it should be 4 kHz.
• GET_SAMPLERATE 0x59: returns sampling rates for SCAN and EXPRESS_SCAN commands
• GET_ACC_BOARD_FLAG 0xFF: seems for accessing A2 motor control board. Not documented.

[jjehl]

Rigth, I need some