Adding support for simple read line with handling of backspace.
Improving support for command line handling; Shell.

1. Refactor shell.run() to use new IOStream::readline() with builtin echo.
2. Update miniterm.py setting in Cosa.mk.
3. Add internal buffer to shell class to allow run to be polling.
4. Removed echo flag in shell class.
5. Clean up example sketches; reduce code.
6. Demonstrate background processing; yield and capture idle time.