Coverage 4.0 support

Seems some things were changed in 4.0:

```
Traceback (most recent call last):
File "/usr/local/bin/duvet", line 7, in
from duvet.__main__ import main
File "/usr/local/lib/python2.7/dist-packages/duvet/__main__.py", line 10, in
from duvet.view import MainWindow
File "/usr/local/lib/python2.7/dist-packages/duvet/view.py", line 14, in
from coverage.parser import CodeParser
ImportError: cannot import name CodeParser
```

Start in duvet/view.py at the import of coverage.parser.CodeParser and reproduce the traceback with the reported coverage version. Check the coverage 4.0 API changes and existing issue discussion; done means duvet starts without this ImportError and can visualize coverage data.

Written by the indexing model from the issue text.