Mass changes in the direction of PEP-8 conformity

Splitting this up into two PRs, as the next batch of changes will take a while. [Progress here](https://github.com/mypaint/mypaint/issues/110#issuecomment-57375349).

Errors remaining:
```
1       E225 missing whitespace around operator
684     E265 block comment should start with '# '
68      E301 expected 1 blank line, found 0
130     E302 expected 2 blank lines, found 1
1149    E303 too many blank lines (2)
268     E501 line too long (>79 characters)
```

The single missing whitespace error is lib/meta.py, which I can't fix as it doubles as a bash script.