Edit files inplace

Inspired by this [blog post](http://www.zopatista.com/python/2013/11/26/inplace-file-rewriting/) and [path.py feature](https://github.com/jaraco/path.py/issues/53).

This adds the Path#rewrite() method, which allows you to do (from docstring and test):
```
with orig.rewrite(read_newline='\n',
                  write_newline='\r\n') as (r, w):
    w.write(r.read())
```

Needs more tests.