Mac OS compatibility (pyionotify not compatible)

Hi! I'd like to give wdb a try but I can't seem to install it on Mac OS X.
It seems that [pyionotify](https://github.com/seb-m/pyinotify) is a Linux-only package.
Is this requirement something you are willing to change?

In any case, it might be a good idea to add this OS restriction to the main README page. ;)

Output from a (Python 3) virtualenv:

```
(myvirtualenv)[11:27:06] rik@rikmbp:[~/projectdir]: pip3 install wdb.server
You are using pip version 7.1.0, however version 8.1.2 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.
Collecting wdb.server
Using cached wdb.server-3.0.6-py3-none-any.whl
Collecting tornado>=4.2 (from wdb.server)
Collecting psutil>=2.1 (from wdb.server)
Using cached psutil-4.3.0.tar.gz
Collecting tornado-systemd (from wdb.server)
Using cached tornado_systemd-1.0.1.tar.gz
Collecting filemagic>=1.6 (from wdb.server)
Using cached filemagic-1.6.tar.gz
Requirement already satisfied (use --upgrade to upgrade): wdb==3.0.6 in /Users/rik/.virtualenvs/myvirtualenv/lib/python3.5/site-packages (from wdb.server)
Collecting pyinotify (from wdb.server)
Using cached pyinotify-0.9.6.tar.gz
Complete output from command python setup.py egg_info:
inotify is not available on macosx-10.11-x86_64

----------------------------------------
Command "python setup.py egg_info" failed with error code 1 in /private/var/folders/99/fdj05zf13tsf59rvsld4753m0000gn/T/pip-build-oj9irrm_/pyinotify
```

Start by inspecting the wdb.server dependency on pyinotify and the installation failure shown in the issue, then check the main README for platform requirements. Reproduce the pip install in a macOS Python 3 virtualenv; done means macOS support is established or the restriction and installation expectations are clearly documented.

Written by the indexing model from the issue text.