improve distribution

This PR reorganizes pathod's installation process:

1. Adjust requirements.txt to remove all inconsistencies (as suggested at https://caremad.io/blog/setup-vs-requirement/). Also, `pip install -r requirements.txt` now installs the latest version of netlib from GitHub.
2. Introduce `extras_require` for dev dependencies (installable via `pip install pathod[dev]`)
3. Use setuptools entry points instead of scripts. Next to being the recommended way, this automatically creates pathod.exe and pathoc.exe executables on Windows.
4. Move `pathod` and `pathoc` into `libpathod.main`. The entry points require a callable function, not a script file.

Change 1 might be debateable. The obvious benefit is that we can advocate `git clone && pip install -r requirements.txt` to hack on mitmproxy - after all, there are a variety of bug reports concerning invalid versions of netlib and so forth. On the other hand, the proposed change of requirements.txt implies that we do not provide concrete dependencies anymore. However, given that `pip install` does not care about requirements.txt, they stay mostly unrecognized anyway.

@cortesi, thoughts?

These changes should be applied to netlib and mitmproxy as well (after reaching consensus).