Some improvements on content and format.

* Names export file 'textpattern-export.xml'.
* Renders valid XML for site name, site slogan, user login names, article titles, commenters' web addresses.
* Exports HTML instead of unprocessed content  for the article bodies and even excerpts. NB: WP seems to allow unfiltered HTML in excerpts yet it does not encourage it through its UI.
* Sets a post's "sticky" flag when applicable.
* Uses curly quotes to fix messy UTF-8 strings.
Merge pull request #1 from rwetzlmayr/horses-mouth

Some improvements on content and format.