added comments regarding possibility of pwm output on pin D4 (PB4)
Added analog & reset functions to pinout diagram
Added variants for 20-pin tinys: x7, x13 & x61 AVR chips
Added README file; fixed incorrect filenames for 20x13 variants
added SPI, I2C constants
fixed README file (was a stub)
Added new configurations to boards.txt & updated README for ATtiny13 support
fixed bad digitalPinHasPWM statement for tiny14 pins