Don't process header/footer files as blog entries

rebuild_all_entries wasn't checking if the file in question being
processed was a header or footer file that could be specified in the
configuration.  Now it checks and skips them if they exist [like the
index and archive file(s)].
Merge pull request #21 from Datyedyeguy/SkipFooterHeaderDuringRebuild

Don't process header/footer files as blog entries