Fixed typo

Corrected spelling of the NUL character (was spelt "null").
Added missing `require'

elwiki-test did not `require' ert.
Added elwiki-vc test and input validation

* `elwiki/log->alist' now throws an error when `commit-log' does not
  contain 4 fields.

* Added an ERT file for elwiki-vc; only has a test for
  `elwiki/log->alist'.
Renamed elwiki-vc test

Renamed log->alist-test to elwiki/log->alist.
Added elwiki test

Added elwiki/page-name test.
Corrected handler mapping in elwiki-page test.
Refactored send-string function

Changed elwiki/send-site-header-or-footer to
elwiki/site-header-or-footer, which only returns the header/footer as
a string for the caller to send, instead of also sending it itself.