Stem's convenience method for saving configuration using SAVECONF

[neena]

I'm implementing the convenience method for saving Tor's current configuration using SAVECONF.

My implementation is ​here (this ​commit). I've based it off my setconf branch because it's using the SingleLineResponse I implemented in it, so this will have to merged after that (which is based on the getconf parsing, because the tests need it). I wish had done this seperately, but, *shrug*.

I also fixed a few things in the documentation which reappeared after a bad rebase to master. such as...

-  :var str code: The error code returned by Tor (if applicable)
-  :var str message: The error message returned by Tor (if applicable) or a human
+  :var str code: The error code returned by Tor
+  :var str message: The error message returned by Tor or a human

[neena]

I've rebased to master, so the link to the commit in the ticket won't work, but it's the same ​branch.

[atagar]

Great change! Pushed - I couldn't think of any bits to twiddle this time. :P

I have added an InsatisfiableRequest class which will

Heh, threw me off for a second since 'insatisfiable' isn't a word. It's right in the change, though.

I've also added a super class for non-socket non-protocol errors called
OperationFailed. Any error that was raised by an error response returned
by Tor (i.e., one with an error code & a message) should subclass this.

Good idea.