examples: added shell over std-IO usage example

In addition to the `test_shell` app I created this little example to demonstrate the usage of the shell without posix-io and uart0. I find it quite useful to demonstrate how easy it is to set up the shell on a new port without any effort.

For this further one question: Is there a reason the shell expects integers as in/out arguments for its callbacks? Otherwise the `init_shell` could be written as: 
`shell_init(&shell, shell_commands, SHELL_BUFFER_SIZE, getchar, putchar);`...