Replace is_valid_email() with filter_var().

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5640 2fea6a4d-a838-0410-917b-93a53c48e9d2
Introduce Textpattern_Server_Var.

Access and handles server configuration variables. Collects
all the tiny snippets from Textpattern so that they can be
used anywhere.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5641 2fea6a4d-a838-0410-917b-93a53c48e9d2