Feature/get dotted columns by prefix
 - [ ] closes #xxxx
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [ ] whatsnew entry

Assuming a DataFrame like this:
```
>> df
    foo  bar.foo  bar.bar
0  0.32     0.15     0.45
1  0.62     0.73     0.36
2  0.21     0.68     0.10
3  0.05     0.36     0.90
```

one can access to `df['foo']` with the shortcut `df.foo`.

This PR extends this idea to column names containing dots, so that `df.bar` yields:

```
>> df.bar
    foo   bar
0  0.15  0.45
1  0.73  0.36
2  0.68  0.10
3  0.36  0.90
```

I'm new to contributing in pandas: so I need a few help:

* Do I need to create an issue to *host* the PR ?
* Where in the docs should I host explainations about this ?

Thanks in advance (and pandas is awesome btw !)