Update task due notification template

* For each task past due, add link to task on kanboard if application_url is specified.
* Add task due date.
* Remove task link on footer.
Update helpers.php, function a()

In helpers.php is a function a() used for link generation, i. e. by markdown(). In the opening <a> element was a /, which is ignored by many (all? most?) browsers, but leads to interesting result with jQuery inserting of HTML generated by that function.

I stumbled upon this while developing code for a preview of textareas and not being able to show a link to a task (#1).
Merge pull-request #356 (bugfix Helper\a())
Add Thai translation (pull-request #353)
Update contributors list
Rename Locales and Templates folders to be more consistent
Update contributor list
Update JsonRPC library
Change Vagrant config to have multiple boxes
Add pagination for users page
Add icons and shortcuts to the board (pull-request #348)
Merge translation file from upstream master