[MRG] ATLAS style: support for multiple orientations and shapes

`./plot_style.py --shape=square atlas`

![ec9166909a9e4c66ba4ef99e0e887f5b](https://f.cloud.github.com/assets/202816/1537772/e79084a8-4cce-11e3-9a3d-e37983015203.png)

`./plot_style.py --shape=rect --orientation=portrait atlas`

![4a348d48c26841e2a9d89b46519fc3fe](https://f.cloud.github.com/assets/202816/1537773/ea03b0f2-4cce-11e3-9040-349dd9da173c.png)

`./plot_style.py --shape=rect --orientation=landscape atlas`

![1cfa9d0f8e2c44be85cde27d6eee9c5d](https://f.cloud.github.com/assets/202816/1537774/ebeebd08-4cce-11e3-9764-13ff00373655.png)