Merge pull request #20 from shengwen1997/master

Finish creating the basic required tasks
merge free_rtos from master
Unify code style
Make LED B blink when enter the flight control task
Change all NVIC setting and remove some interrrupt relative initial
TODO: move these ISR-ralative initialization function to a task
Because we will use FreeRTOS API to deal with the ISR,
It is the better way that check it in FreeRTOS task.
Change task name length and MAX_PRIORITIES to 10
Disable FreeRTOS ISR API

We should call FreeRTOS ISR API after start scheduling.
I will deal with it later
FreeRTOS could start scheduling now
TODO: Seperate correction and flight control function
We still cannot enter flight control now
Move the System Initialization out to be a independent task

*The other task will running after system finish initializing, otherwise the task will be freezed