Moved TextpatternLoader to its own file.

Drops dependencies and allows calling the loader before
anything else. Renames the class to Textpattern_Loader
in the process.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5342 2fea6a4d-a838-0410-917b-93a53c48e9d2