Enable command for serial shell and remote shell

This pull request is a fix of #15 

1. We add new app -- remote_shell.c. This app is similar to serial-shell but handles command received via wireless communication

2. Enable following commands:
* reboot
* blink
* txpower (query/change TX power)
* rfchannel (query/change RF Channel)
* nodeid (query/change node id) (this function requires the fix of #21 )

3. add new command:
* sendcmd <command>

This new command allows us to send command to remote note via serial shell (type the input and then send)

Please see reboot_exampe for how to use this feature.