Move export_wp() to an include file

[tellyworth]

This is a simple refactoring job that makes it possible to call export_wp() for unit testing. The function now lives in an include file, and it no longer calls die() at the end.

[ryan]

How about passing author_id to export_wp() instead of it relying on _GET?

[tellyworth]

pass author as a function parameter

[ryan]

(In [6335]) Move export_wp() to an include. Props tellyworth. fixes #5347

[darkdragon]

Shouldn't this be moved to 2.4, since it is trunk and this was fixed on the trunk?