usart tests thoroughly refactored. all test data are now allocated on heap using factory method.
Merge pull request #8 from matianfu/master

usart tests thoroughly refactored.
new struct in usart.h
usart irq data struct fixed. global irq registry used.
UARTEX handle ctor updated.
ctor/factory updated for new uart_ops field in uartex struct.
add test cases for uartex factory, invalid args.
backup. refactoring uartio.
uart io read test refactored, cleaned.
some dummy changes for lectures.
Merge pull request #10 from matianfu/master

merge develop branch
add comments
Merge remote-tracking branch 'upstream/master'