Set up client

[afuetterer]

Let's begin with the basic functionality of the client:

• BaseClient is ABC
• Client (sync)
• Base URL
• User agent
• Timeout
• _request()
• add tests (vcr.py)
• list()
• get()
• 404
• Add mini usage example in readme
• add Client to api reference docs
• Returns the xml for now
• Argument return response or xml, return_type. Defaults to xml.

import re3data

re3data.repositories.list()
re3data.repositories.get(zenodo)

[afuetterer]

async, filtering, counting, parsing of xml will come next