More options

This PR adds a bunch of options to make using etcdtar easier:

 * Use `--path=<path>` to only export a sub path. Fixes #1 
 * Use `--weak-consistency` to use `etcdtar` with a single-node cluster (I needed this to talk to my one-node etcd)
 * Added two debug flags: `--show-curl` and `--debug-etcd` to print etcd-go internal debug output