Make automatic variable imports in EquationWriter optional.

[schymans]

This enables deactivating automatic inclusion of variable imports by EquationWriter. The default is still to include all variables, but it can be deactivated by passing autovars=0 to EquationWriter.eq and EquationWriter.neweq.

Closes#100.

[schymans]

For some reason, sphinx fails in the travis-ci:

Exception occurred:
File "/home/travis/virtualenv/python3.6.7/lib/python3.6/site-packages/nbsphinx.py", line 1813, in apply
env.get_domain('math').data['has_equations'][env.docname] = True
KeyError: 'has_equations'
The full traceback has been saved in /tmp/sphinx-err-vakrs_zm.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!
The command "python setup.py build_sphinx" exited with 1.

This does not happen locally for me. Any idea what might be wrong?