Hepdata-converter-ws

Minor update to only catch exceptions from requests with status code < 500. This recovers the behaviour from before the last 0.1.4 release, where an HTML file with error details is returned for a failed conversion.

* This release adds Python 3 compatibility, while still maintaining Python 2 compatibility.
* The tests have been improved: they no longer depend on the `hepdata_converter` module and now run outside the `hepdata-converter-ws` Docker container, testing the web service connection.
* A `timeout` parameter (with default of 600s) can be passed as an argument of the `convert` function.
* Error handling has been added to the request to `/convert`.
* Dependencies have been moved from `requirements.txt` to `setup.py`.
* Documentation has been improved in the `README.md` file, in the docstring of the `convert` function, and in the metadata pushed to PyPI for tagged releases.

- Added ping to test connection