remove detailed proxy status

There are two problems with obtaining the proxy status via the normal
proxy endpoint...

1. when TLS is enabled, this requires using TLS at the client end

2. the status URL is mixed in with the docker remote API, which is
rather unclean

There is limited value to having some status output from the proxy,
since all the output is constant.

So instead we log the version and command line args, and in `weave
status` only report whether the proxy is running.

We still want a way to wait for the proxy to start up, and log a nice
error if it dies. We do this by polling the docker logs for a startup
message.

Fixes #877. Fixes #876.

Undoes most of #756, #800, #854.

site/proxy.md

@@ -206,16 +206,11 @@ The command
     weave status
 
 reports on the current status of various weave components, including
-the proxy:
+the proxy, if it is running:
 
 ````
 ...
-weave proxy git-03ede29e7ee4
-Listen address is :12375
-Docker address is unix:///var/run/docker.sock
-TLS off
-DNS off
-IPAM off
+weave proxy is running
 ````
 
 Information on the operation of the proxy can be obtained from the