Clarify dependencies

The purpose of this pull request is to:
- [x] Clarify in the README which obligatory and optional dependencies CONCOCT has.
- [x] Switch to anaconda as the recommended way of installing python dependencies
- [x] Switch to miniconda as the way of installing dependenvies on travis
- [x] Have a complete list of python dependencies in setup.py so pip can install CONCOCT through pypi.
- [x] Add dependencies used for travis.