Support generating bash completion shells

[nak3]

• This patch includes completion sub-command for torusctl and torusblk and completionoption for torusd to generate bash completion shells.

[nak3]

Usage example

$ export PATH="$GOPATH/src/github.com/coreos/torus/bin/:$PATH"

$ source <(./bin/torusctl completion)
$ source <(./bin/torusblk completion)
$ source <(./bin/torusd --completion)

$ torusctl <TAB> <TAB>
block       completion  config      init        list-peers  peer        ring        version     volume      wipe

$ torusctl --<TAB> <TAB>
--config=            --etcd=              --etcd-cert-file=    --profile=           --read-level=        --write-level=       
--debug              --etcd-ca-file=      --etcd-key-file=     --read-cache-size=   --write-cache-size=