Convert to BusDevice

[caternuson]

For #8

Basic check:

Adafruit CircuitPython 4.1.0 on 2019-08-02; Adafruit ItsyBitsy M4 Express with samd51g19
>>> import board
>>> import adafruit_mcp4725
>>> dac = dac = adafruit_mcp4725.MCP4725(board.I2C())
>>> dac.value = 0
>>> dac.value
0
>>> dac.value = 65000
>>> dac.value
64992
>>> 

More fun check:

import time
import math
import board
import adafruit_mcp4725

dac = adafruit_mcp4725.MCP4725(board.I2C())

theta = 0
dt = math.pi / 100

while True:
    dac.value = int(32767 * (1 + math.sin(theta)))
    theta += dt
    time.sleep(0.01)

[ladyada]

nice n ez, needs mock i think?