Pybbm

* Hot fixes to bbcode transform

* bbcode engine simplified

* Pybbm specific forms moved to views' attributes, added new functions to views to get such forms dynamically.
This makes overriding pybbm forms much easier
* Moving from unmaintained postmarkup package to bbcode project as default bbcode render engine
Changed output html for [code] tag. It will be <code></code> tags instead of <div class="code"></div>.
So you should duplicate styles applied to div.code for code html tag.
* Japanese translation

* Hot fixes for Python 3 support
* Fixes for Chinese translation

* Python 3 support
* Chinese translation

* Two new methods added to permission handler: `may_attach_files` and `may_create_poll`. First method used for
restrict attaching files to post by user. By default it depends on `PYBB_ATTACHMENT_ENABLE` setting.
Second may be used to restrict some users to create/edit polls. By default it always return `True`.
For disabling polls on your forum, just write custom permission handler and return from this method `False`