Adding support to switch from Textile to Markdown

This adds a checkbox in the advanced section to allow selecting Markdown as the syntax for release notes. It will switch the parameter `notes_type` to _1_ if selected, else _0_ is being used as before this change. You can see more about the `notes_type` parameter in the [HockeyApp API](http://support.hockeyapp.net/kb/api/api-versions).

The value of the checkbox is being ignored when change logs are used as release notes.