Add simple countdown feature for timer 0.

Extends the "delay" feature with ability to asynchronously wait and execute
other operations while waiting.

This code has been in use already some time and with two different APIs. For this reason the code
is wrapper with two flags: EXTRA_TIMER_0_COUNTER and TWO_COUNTERS

EXTRA_TIMER_0_COUNTER enables the old, single counter API.
Also defining TWO_COUNTERS selects the new API with two independent counters.

(This is still a temporary solution until timer 0 will be disabled permanently)
Add MemoryFree library.
Disable deprecated SD library functions.
Disable SD library's empty File class destructor to prevent linker errors.
Fix compilation warnings and errors on SD library.
Add header file for common abstract serial communication interface.

Current S/W and H/W serial implementations have somewhat different APIs. This
interface ensures a common API at least for the basic features.
Align S/W serial logic and naming with H/W serial, inherit common serial interface.