etcdserver/etcdhttp: store location adjustment

Detailed adjustment:
/_etcd/machines/* -> /0/members/*
/* -> /1/*

And it keeps key path returned to user the same as before.

fixes #1331
@philips i cannot find a clean and easy way to make it in the store:
1. all public API in store needs to add a prefix argument, or introduce some `SetPrefix()` for store. both ways are terrible. The public API in store has had too many arguments yet, and `setPrefix` adds an indirection still.
2. the key stored in store.node is in the format of full path. we need to do string trim anyway.

Another alternative that I have tried is to wrap Store to make it more readable. But that is terrible. `Watch()` returns a `chan *Event`, and i cannot wait on the channel, trim each event and push it in a new channel efficiently in a wrapper.

So I choose this solution at last.