Added system timer class.  From testing on the stm32f407 I had to change the HSE_VALUE constant in stm32f4xx.h from 25Mhz to 8Mhz to get the timing to work correctly.  I'm not sure if we'll have to mess with this when we move to the f405.   Sorry about the whitespace bloat.  If stm had a half-way decent coding standard this wouldn't happen.  I'm too lazy to revert and retest.