Implement `FromStr` for `Request`

[TTWNO]

I wanted to add an example that allows you to write raw SSIP commands and talk to the sever, and then maybe even a tab-completion style client.

However, this seems to be much harder than I thought since there is no way to send raw text with our API, and there is also no way to convert from a string to a Request.

This is mostly just for user ability stuff; it isn't really necessary for the protocol.

So there are two parts:

• impl FromStr for Request
• make raw command example that replies in raw or debug'd text