Merge pull request #3 from guluguluchui/BeagleBoneBlack

Merge Beagle bone black into master. Add support for Beagle bone black board and provide analogRead() funciton (reference voltage = 1.8V)
Merge pull request #4 from guluguluchui/BeagleBoneBlack

Beagle bone black
Update support for BBB and docs
Update doc
Try to control gpio by writing registers instead of /sys/...
On BeagleBoneBlack ,speed up digitalRead() & digitalWrite().
Change to the way directly writing or reading gpio registers.
Reduce digitalWrite() excution time.