PlatformIO Library Registry manifest file

* This library in Web Registry: http://platformio.ikravets.com/#!/lib/show/106/I2Cdevlib-MPR121
* Specification: [PlatformIO Library Manager](http://docs.platformio.ikravets.com/en/latest/librarymanager/index.html)
* Integration: [IDE Integration](http://docs.platformio.ikravets.com/en/latest/ide.html)
Merge pull request #115 from ivankravets/patch-15

PlatformIO Library Registry manifest file