Add stub clientapi webserver with readers/writers packages

[kegsay]

This automatically does JSON and logs with request IDs because it uses matrix-org/util. It looks like:

$ BIND_ADDRESS=:7878 ./bin/clientapi 
INFO[0000] Starting clientapi                           
INFO[0005] Incoming request                              req.id=x1slkZ1Mrk3B req.method=GET req.path="/sync"
INFO[0005] Doing stuff...                                req.id=x1slkZ1Mrk3B req.method=GET req.path="/sync"
INFO[0005] Responding with error                         error="Not implemented yet: 404: " req.id=x1slkZ1Mrk3B req.method=GET req.path="/sync"
INFO[0009] Incoming request                              req.id=ioZU9NanGplb req.method=GET req.path="/send"
INFO[0009] Doing stuff...                                req.id=ioZU9NanGplb req.method=GET req.path="/send"
INFO[0009] Responding with error                         error="Not implemented yet: 404: " req.id=ioZU9NanGplb req.method=GET req.path="/send"

Obviously the endpoints are stubs for now, but this gives the skeleton for how new APIs will be added.

[NegativeMjark]

LGTM