make AMCAT pip-installable

* README updated to reflect this
* manage.py is now a package with an __main__ package, run with python -m
  (see README)
* setup.py is not installed along, so version info encoded in amcat._version

TODO:
* there's now `amcat.manage` and `amcat.management`, maybe merge these or rename one of them?
* `pip install` installs far too many packages that are not under the `amcat` namespace (e.g. `navigator`). It would be nice to move these under `amcat`.