minor optimizations
implicit initializations
Merge pull request #1 from duartebarbosa/optimizations

Optimizations
Verified that NULL is a 0 in AVR.
Reapeating Duarte's optimizations in time.c through the rest of the module.
Harmonized style throughout the code.
Substituted SPDT switch for a SPST with a pull-down resistor.
Minor optimizations using inline and static. Cleaned code in led sysmod.
Corrected bug on PWM ports initialization.
Corrected bug in ADC. Was waiting for bit set instead of clear.
Added functions to change the ADC reference voltage.