Added support for 1Mhz, 8Mhz and 16Mhz frequencies for ATTiny85, ATTiny84 and ATmega328

Library now supports following chip frequencies for RX
ATTiny85  @ 1Mhz, 8Mhz, 16Mhz
ATTiny84  @ 1Mhz, 8Mhz, 16Mhz
ATMega328 @ 8Mhz, 16Mhz