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
Merge pull request #42 from RLovelett/multiple-headers

Support Multiple headers