ENH montage:  add 2d grid option

First step following up on discussion in https://github.com/nipy/PySurfer/pull/79 to add to make_montage the capability to create 2d grids

```python
import surfer

subject_id = 'fsaverage'
std_args = [subject_id, 'lh', 'inflated']
small_brain = dict(size=100, width=500, height=500)

brain = surfer.Brain(*std_args, config_opts=small_brain)

brain.save_montage("test_2d.png", [['l', 'v'], ['m', 'f']])
```
![test_2d](https://f.cloud.github.com/assets/145771/1951507/c9295d86-8181-11e3-9985-aa20ac85e3bb.png)