Fix the Windows support

[liZe]

I've tried many little things to make WeasyPrint work with both Python 2 and 3, and here's the best solution I've found.

• As explained in Fix Python 3 compatibility on Windows #132, the problem is that pathname2url doesn't accept bytestrings with Python 3 on Windows. As Python 2 allows bytes and Python 3 allows unicode with all OSes, I've added the encode in the compat module's pathname2url implementation.
• Contrary to what I've said, there's no need to be afraid of returning unicode strings in path2url: unicode is actually already returned because of the implicit cast due to u'abc' + b'abc' at the end of the function.
• @SimonSapin's idea to use open instead of urlopen is a good idea, but it's not related to this problem. For the record, I've tried to use open, but URIs for Windows filenames have an extra slash before the volume (something like /C:/path). This leading path is explained by the RFC and it's handled correctly by urlopen, duplicating code to handle it (and other needed features such as guessing mimetypes) in WP may be useless. We can do this later if we want.
• I've put the FILESYSTEM_ENCODING variable in the compat module: it's not a bad place and avoids a circular import.

Finally, this PR is pretty equivalent to #132, but a bit cleaner 😄.

It's been tested with Python 2.7 and 3.5, on Windows 10 and Linux. Some tests launched with py.test even work on Windows (but some tests make Python crash, fun fun fun).

[SAPikachu]

Thanks! Just tried and it seems to work fine here.

[liZe]

Let's go!