Find coverage during configure, issue with Debian package

- Debian Squeeze and up have a package named “python-coverage”, but it
  doesn't use the same binary name as upstream (“coverage”).
- Said package includes a patch to use symlinks instead of file copies
  for jQuery. If files from previous runs are around, an exception is
  raised. This is fixed by removing all regular files and symlinks
  before generating a report.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>