Fix building the documentation on readthedocs

[adamreeve]

For some reason importing nptdms by modifying sys.path stopped working in readthedocs. The current readthedocs documentation recommends installing the package with pip in the readthedocs python configuration, which fixed the issue.

This also fixes a few other things like adding the build.os configuration option which is now required, and failing the builds on warnings so that issues like this are more visible in future.

Fixes #309