Support Multiple headers

Sections can support multiple headers

Each section can now have a single header (previous behavior). It can now also support multiple headers. The spec defines three types of headers: `default`, `first`, and `even`. These code changes provide support for all three.

The HeaderFooter.php example has also been updated to illustrate how to use the new multiple headers for a section.

Used these pages as spec reference:

http://www.schemacentral.com/sc/ooxml/e-w_headerReference-1.html
http://www.schemacentral.com/sc/ooxml/a-w_type-4.html

Should provide the feature described here: https://phpword.codeplex.com/discussions/270211. Better late than never! :thumbsup:

(Orignally submitted in pull request #15)