Migrate from PIL to Pillow

[Pillow](http://pillow.readthedocs.org/en/latest/) is a friendly and maintained fork of PIL.

I have primarily created this branch because of the recent changes in [pip 1.5](http://www.pip-installer.org/en/latest/news.html#id1). Latest version of pip does no longer scrape insecure external URLs, which means you get the following error when installing mitmproxy using pip v1.5:

```bash
...
distributionacking PIL (from mitmproxy)
  Could not find any downloads that satisfy the requirement PIL (from mitmproxy)
  Some externally hosted files were ignored (use --allow-external PIL to allow).
  Some insecure and unverifiable files were ignored (use --allow-unverified PIL to allow).
No distributions at all found for PIL (from mitmproxy)
Storing debug log for failure in /home/kami/.pip/pip.log
```

You can ignore this error and force pip to install files from external URLs by passing `--allow-external PIL --allow-unverified PIL` flags, but that's obviously not secure or recommended.

Even without this change, using Pillow is generally more recommended, because unlike Pil, Pillow is actively develop and maintained.

Pillow is more or less drop-in replacement and the console_contentview tests pass.