Make command replies consistent with requests

As talked before it would be nice to have command replies (for example !bat) to answer with some consistency to command requests for easier handling by software.

As proposed before each command reply can start with the character `$`.

For example when requesting battery info through `!bat` the reply from the freakwan device could be `$battery 100%, 4.20 volts`

In this way we could avoid using for example regexps to handle every reply from the node and easily distiguish between command replies and received messages (from other freakwan nodes)