Added functions getDeviceFriendlyName() and getDevices(). 

getDeviceFriendlyName
Argument: Serial Number
Returns the device's friendly name if set, else returns DEVICE_WITH_NO_NAME

getDevices() - Useful in multi-device scenarios. Returns an array of serials found.
added serial_number check as proposed by gboudreau
Merge pull request #19 from rkaramchedu/master

Added two new functions