detect docstrings and treat them as such

This checks for the first unbroken line of comments
directly following a function declaration,
and hoists them into a docstring.

For example, with the input file `test.m`:
```matlab
function y = test(x)
% This is a test function.
%
% Usage:
%   y = test(x);

y = x;  % magic
```

We get:
```python
# Autogenerated with SMOP version 0.23
# smop/main.py test/test.m -o test/test.py
from __future__ import division
import numpy as np
from scipy.io import loadmat,savemat
import os

def test(x):
    """This is a test function.

     Usage:
       y = test(x);
    """
    y = x
    # magic
    return y
```