Inject: simplify inject, remove unnecessary code
RefProxyList: list proxy for references

As needed for sources of EntityWithSources
EntityWithSourcesMixin: inject sources list

- Add mixin that enhances all classes that derive from
  EntityWithSources
- Write test for sources list of DataArray
- This fixes #13
Merge pull request #18 from stoewer/with-sources

Implement mixin for EntityWithSources

LGTM
Docs: stub for sphinx documentation
Feature first draft
inject: better meta class behaviour
PyEntityWithSources: splitt up mixins
core: configure doc strings
docstrings: define docstrings in a central header
setup: configure sphinx build

- build dox with
  python setup.py build_sphinx
PyEntity, PyBlock: use docstrings in bindings
PyBlock: full documentation example
Merge pull request #19 from stoewer/documentation

Documentation using sphinx

LGTM
wrote SimpleTag and Feature, tests needed
tests for SimpleTag done
tests for the feature added
merged changes for docs