Implement a poorman's printf for Arduino

In error.h (or perhaps somewhere in compat, I dunno) we could write a "poorman's printf" that would just iterate through the %-style arguments and call Serial.print() on them. (Actually it should rather be a poorman's version of vsnprintf()). There could also be a flag that would allow one to switch (ie. use stdio printf if you feel like you can afford it memory-wise).
