This updates to an anaconda python image and removes redundant tests
rerunning examples so that the tests finish in a more reasonable amount
of time. Many requirements are also moves to an
optional_requirements.txt file.

- Adds in comments from Denyz, and general tidy up.
- Closes #83

- Moves the hyperPE likelihood to core
- Adds the GaussianLikelihood
- Update docs
- Update examples

- Convert markdown readme to rst
- Add pip install entry to installation instructions

- Move instructions to docs
- Update and simplify readme
- Add note on HTTPS

Previously I was setting the noise_log_likelihood to nan in a confusing
way. This removes that and also renames all the liklihoods for clarity.

- Add doc file on CBC PE
- Add introduction text

An issue with html5lib upstream is causing the docs to not be generated.
This ensures a version is used that will make the docs.