TinyMCE: improve handling of Read More and Nextpage tags. See #24067, fixes #16239.

git-svn-id: http://develop.svn.wordpress.org/trunk@26941 602fd350-edb4-49c9-b593-d223f7449a82
TinyMCE: add/remove the 'alignnone' class when aligning images without captions. See #24067.

git-svn-id: http://develop.svn.wordpress.org/trunk@26942 602fd350-edb4-49c9-b593-d223f7449a82
Remove unnecessary sprintf(). props jdgrimes. fixes #25489.

git-svn-id: http://develop.svn.wordpress.org/trunk@26943 602fd350-edb4-49c9-b593-d223f7449a82