Move extra package requirements to a proper place

Everything can still be installed by pip:
$ pip install -e .[shell,dev]
for development and
$ pip install -e pyshop[shell,wheelify]
for releases.

See [1] and [2] for details.

[1] http://pythonhosted.org/setuptools/setuptools.html#declaring-extras-optional-features-with-their-own-dependencies
[2] http://pythonhosted.org/setuptools/setuptools.html#declaring-dependencies