ADDED ENTRY-HEADER.PHP

This is the first pull request I've ever done in my whole life, so please let me know whether I'm doing this correctly.

With this commit, I've created ``entry-header.php`` to mirror the already existing ``entry-footer.php`` and I've used it in place of several places, whenever ``<header class="entry-header">`` appears at the beginning of an entry.

This is related to Issue #17