improve ex_utils python module

We add these features:
- adjustable system frequency
- access to system clock object
- enable tracing
- enable gdb (only partially working)
add support for fractional clock periods, e.g. 62.5ns for 16Mhz
add support for U2X bit in UART
add more subsystems to Python wrapper
try to avoid sending '\0' char when UART is enabled
change behavior of UART to match a real ATmega328
fix crash if name of trace file doesn't contain a dot