New pin configurations & features to ATtiny package

I have added the following features that would be useful to integrate into your main repo:

1) I have added support for 20 pin ATtiny chips. Specifically:

ATtiny87/167
ATtiny2313/4313
ATtiny261/461/861

2) I have also added the ATtiny13 to boards.txt.

3) Added SPI & I2C support to all the pinout files

4) Added readme file to provide guidance on installing and using.

Please consider merging these changes as you are able.

Thanks,
Scott Daniels