pull in changes from numbered_headings plugin
re enable 'preview' in wiki#edit

- fixed JS check for tinyMCE
- before clicking on preview without tinyMCE raises an JS error and preview was not working
fixed textilizable helper method, wiki#edit

- method returns not formated text on edit if xml formatting is disbaled
- indentation fix
Merge pull request #52 from opf/feature/numbered_wiki_headings

pull in changes from numbered_headings plugin