Added register bits for SysTick
Added SysTick methods and implementation
Added basic RTC functionality
Added set/reset backup domain
Added interrupt functionality to USART/UART
Added clock configuration specific to stm32f4-discovery board
Fixed comments
Added SysTick demo in f4discovery
Renamed  misleading delay_1_second
Added RTC demo to f4discovery
Merge branch 'master' of https://github.com/jaqueza/libstm32pp