Compatibility to stm32f4
Support for stm32f4 added from @ramboerik (See #75)
The feature requires the latest release of arduino core from STM (version 1.9.0) and is only verified for stm32f4 but could probably run on stm32fX with minor adjustments.
There is one minor bug that causes the first step interrupt to be triggered at slightly wrong time but that can be fixed later on.