add -tree option to images

This adds another option to `images` to print the image hierarchy in text form.

```
$ docker images -tree
|─27cf78414709
| └─b750fe79269d Tags: ubuntu:12.10,ubuntu:quantal
|   |─15fcc3627e79
|   | └─4b6446d47a37
|   |   └─0c074eec5f5c
|   |     └─3d546eda2db2
|   |       └─d660c80f5e1f
|   |         └─596fb3a686cf
|   |           └─61517a9129a2
|   |             └─3ad08d106530
|   |               └─64c0a2fd7c30
|   |                 └─1fde87bad242
|   |                   └─63b2b9368988
|   |                     └─fc4708bf64c3 Tags: ndj/mongodb:latest
|   |─3e11a97c3f82
|   | └─9dc7a7486e87
|   |   └─b179e38c4afd
|   |     └─b98d65ae00f7 Tags: shykes/znc:0.1,shykes/znc:git-eb9ffb538d1539c41efd03fa66c5ac1b8b91ca2a,shykes/znc:latest
|   └─8ee958cf9d12
|     └─adcb0fb35213
|       └─a6ddc80ec783
|         └─cc7feb9c3b51
|           └─b28ecfd506c6
|             └─37e7fb8b1be1
|               └─a22891c05c50
|                 └─cf9194b5b2b0
|                   └─564bac8b57aa
|                     └─e319e5b173bf
|                       └─c4551a6f0379
|                         └─b7bf889a949b
|                           └─ed37dbb60f27
|                             └─d210ec474616
|                               └─6311f2fa7ca9 Tags: ndj/redis:latest
|─8dbd9e392a96 Tags: ubuntu:12.04,ubuntu:latest,ubuntu:precise
| |─4df3eeb2c202
| | └─dd5498b6c712
| |   └─8d04c42f264e
| |     └─0c573a4b40cf
| |       └─a35fd99a7ea9
| |         └─dde68970d84b
| |           └─1822dc1f204e
| |             └─d45e5ba6dc2f
| |               └─d95940e2b97c
| |                 └─ee69859f33e8
| |                   └─29c4a226db51
| |                     └─df6178eb4c47
| |                       └─d809b136c558
| |                         └─0e19c79ffe35
| |                           └─86d4dd50dd90
| |                             └─aa6181c15178
| |                               └─c3fb316e3819
| |                                 └─f7ee6b773e3b
| |                                   └─c59ea12f6979
| |                                     └─e4e095dc03e5
| |                                       └─7e953d9b8602
| |                                         └─fb2759a6fbb5
| |                                           └─254835df9ccc
| |                                             └─e3fd1ccf8a1d
| |                                               └─115de9124bf2
| |                                                 └─3387c441027a
| |                                                   └─6bde376f8cc1 Tags: docker:latest
| └─d2d41694c20c
|   └─4cede2d58460
|     └─8d5d054f1376
|       └─5ade2d9ec9e0
|         └─76b0b1ab5fd5
|           └─ac3cca0eb14d
|             └─81acc6d83075
|               └─d31c015a3240
|                 └─c1c53577dc84
|                   └─2343dfdcc852
|                     └─95f9da0b614d
|                       └─2d6802031bf9
|                         └─b32035a894f2
|                           └─86858040aa36
|                             └─2ef244b028ac
|                               └─020b93c719a5 Tags: ndj/hipache:latest
└─e9aa60c60128 Tags: docker-ut:latest
$
```

This would (hopefully) solve #1776.