Create library.json for PlatformIO

[solon]

I thought it would be useful to have this library available through the PlatformIO Libraries Registry so I went ahead and added a library.json file for this purpose. If you want to merge it, the library can then be registered from the command line as follows:

platformio lib register https://github.com/wayoda/LedControl/blob/master/library.json

See http://docs.platformio.org/en/latest/userguide/lib/cmd_register.html for details.