Need BH1750 sensor library

[terrillmoore]

The BH1750 library we've been using is really inadequate for our purposes. We want a library with following features:

• better initialization (using static rather than a dynamic constructor)
• by default, non-continuous measurements
• able to select any of the three resolutions for any given measurement
• inherit from cPollableInterface so that we can start a measurement and then get a callback when its ready rather than having to busy spin for 120 ms or whatever
• the BH1750 varies from .96 to 1.44 lux/bit, a range of +/- 20%. With calibration, the result can be trimmed to make measure m the default is 1.2, but support for calibration (via data stored in FRAM) would let us trim this probably to 5%.
• Since the measurement period can also be adjusted, we should be able to optimize flicker rejection for indoor lighting applications, if we calibrate the measurement period.
• avoid hanging on I2C bus (this may be a problem with the current Wire library, but since we have our own BSP, we can possibly upgrade).

[terrillmoore]

Some of these problems have been fixed in our fork of the BH1750 library. Perhaps we should now pull those into the core library with the above changes.