API for selecting ranged groups
I often find myself writing code that looks like this:

``` python
start_group = df[df.str_col == 'start'].index.values + 1
end_group = df[df.str_col == 'end'].index.values - 1
for s, e in zip(start_group, end_group):
    some_function(df[s:e])
```

to get frame subsets between repeated start and end indicators from a particular column.

Possible API:

```
df.grange(marker_col='str_col', start='start', end='end')
```

which would return an iterator over the the subsets.

I'm not sure if this is general enough to include in library code. I'll happily close the issue if it's not. There also might be a way to do this already.
