add tox configuration for testing convenience

the settings regarding used directories are necessary so the buildout ends up in tox's `{envdir}`
`setuptools` must not be pinned

this is because installing `zc.buildout` via `pip` will likely fetch a version newer then the one specified by plone kgs, which in turn causes a version conflict
Merge pull request #38 from witsch/tox-integration

tox integration for testing convenience