Add category to library.properties
Add category to library.properties

Fixes the `WARNING: Category '' in library BLEPeripheral is not valid.
Setting to 'Uncategorized'` warning in Arduino IDE 1.6.6.

femtoduino/arduino-BLEPeripheral,sandeepmistry/arduino-BLEPeripheral,femtoduino/arduino-BLEPeripheral,sandeepmistry/arduino-BLEPeripheral,sandeepmistry/arduino-BLEPeripheral