Implement email encoders under Textpattern_Mail_.

Marks encode_mailheader() deprecated, eventually
freeing bit of the global namespace.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5502 2fea6a4d-a838-0410-917b-93a53c48e9d2
Visual confirmation when changing default section.

Shows error message on error, and success otherwise.
Now also validates the given section name.

Fixes issue #384

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5503 2fea6a4d-a838-0410-917b-93a53c48e9d2
PSR naming convention.

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5504 2fea6a4d-a838-0410-917b-93a53c48e9d2
Textpattern_Mail_Encode::header() throws exceptions.

See issue #385

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5505 2fea6a4d-a838-0410-917b-93a53c48e9d2
Merge remote-tracking branch 'upstream/master'