Enforce use of spaces for indentation and cleanup whitespace (#353)

* Enforce use of spaces for indentation

We cannot just assume that each and every contributor has set
indent-tabs-mode to nil, and indeed it appears at least a few
have not done so in the past.

* Fix indentation

* Fix excessively long lines

I tried to stay away from only slightly too long lines, but did not
succeed completely.

* Cleanup whitespace

Author: tarsius