Fix typo in documentation
Enable global interrupts in the application

The user must enable global interrupts in their application after
initializing all libraries.

http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=141198
Optimize getting data from queues
Make access to the TX queue atomic

There is a race condition when the application is writing data while the
RX interrupt is echoing input.

Also remove blocking/non-blocking option. The blocking functions now
call the non-blocking functions.
Fix bug with erasing characters

When cond is is_not_space and RX queue is empty, an extra character
would be erased in the TX queue.