Add Timer/Counter3 support, fix issues #223 and #225 for several controllers

Changed / fixed definitions for at90can128(/64/32), atmega128, atmega64, atmega162 and atmega1284p.
* Added e6-style definitions to encapsulate access to the second 16-Bit timer Timer/Counter3 on e6-supported controllers providing it.
* Fixed bitmask values for TCn_PRESCALER_xy (issue #225).
* Fixed use of wrong register TCCR1 instead of the register pair TCCR1A/B (issue #223).
* Provided compat-define for MCUSR for atmega162 (issue #223).

Issues #223 and #225 remain open with other controllers definitions.