Initial implementation of step-wise execution for #5

phases)

```clojure
(use 'pallet.repl)
(use 'pallet.api)
(use 'pallet.actions)
(require '[pallet.crate :as crate])

(def c (compute-service :aws))

;; initiate session
(start-repl-session c {:image {:image-id "us-east-1/ami-89181be0"}})

;; do a mock run of a plan function
(explain-step (plan-fn (exec-script "ls -la /")(crate/assoc-settings :mongodb [:cluster false])))

NODE: [repl-session-36fc6e65 repl-session 54.82.169.150 :ubuntu :os-version nil :id us-east-1/i-36fc6e65]
ACTION: pallet.actions/exec-script* of type script executed on target
  FORM:
    (pallet.actions/exec-script* "ls -la /")
  SCRIPT:
    | ls -la /
nil

 (step-lift (plan-fn (exec-script "ls -la /")(crate/assoc-settings :mongodb [:cluster false])))
12:35:49.540 [operate-96] INFO  pallet.ssh.execute - 54.82.169.150 22
SESSION:
NODE: repl-session-36fc6e65 IP: 54.82.169.150 GROUP: :repl-session
nil

PHASES: configure
GROUPS: repl-session
ACTIONS:
  PHASE configure:
    GROUP repl-session:
      NODE 54.82.169.150:
        ACTION ON NODE:
          SCRIPT:
          | #!/usr/bin/env bash
          | set -h
          | ls -la /
          | exit $?
          EXIT CODE: 0
          OUTPUT:
          | total 84
          | drwxr-xr-x 22 root root  4096 May 29 18:08 .
          | drwxr-xr-x 22 root root  4096 May 29 18:08 ..
          | drwxr-xr-x  2 root root  4096 Feb 26 03:35 bin
          | drwxr-xr-x  3 root root  4096 Feb 26 03:35 boot
          | drwxr-xr-x 13 root root  3920 May 29 18:08 dev
          | drwxr-xr-x 89 root root  4096 May 29 18:09 etc
          | drwxr-xr-x  4 root root  4096 May 29 18:09 home
          | lrwxrwxrwx  1 root root    34 Feb 26 03:35 initrd.img -> /boot/initrd.img-3.11.0-17-generic
          | drwxr-xr-x 20 root root  4096 Feb 26 03:34 lib
          | drwxr-xr-x  2 root root  4096 Feb 26 03:33 lib64
          | drwx------  2 root root 16384 Feb 26 03:35 lost+found
          | drwxr-xr-x  2 root root  4096 Feb 26 03:33 media
          | drwxr-xr-x  3 root root  4096 Jan 13 07:37 mnt
          | drwxr-xr-x  2 root root  4096 Feb 26 03:33 opt
          | dr-xr-xr-x 85 root root     0 May 29 18:08 proc
          | drwx------  3 root root  4096 May 29 18:08 root
          | drwxr-xr-x 17 root root   640 May 29 19:35 run
          | drwxr-xr-x  2 root root  4096 Feb 26 03:35 sbin
          | drwxr-xr-x  2 root root  4096 Feb 26 03:33 srv
          | dr-xr-xr-x 13 root root     0 May 29 18:08 sys
          | drwxrwxrwt  4 root root  4096 May 29 19:35 tmp
          | drwxr-xr-x 10 root root  4096 Feb 26 03:33 usr
          | drwxr-xr-x 13 root root  4096 Feb 26 03:35 var
          | lrwxrwxrwx  1 root root    30 Feb 26 03:35 vmlinuz -> boot/vmlinuz-3.11.0-17-generic
SESSION:
NODE: repl-session-36fc6e65 IP: 54.82.169.150 GROUP: :repl-session
{:mongodb {nil {:cluster false}}}
```