Halt action hooks

Call every halt_action present in $_GET using do_action on init.
Added some misc functions
Added Halt Import/Export functionality
Merge $_GET and $_POST functions into one
Halt admin action hooks
Hook tools page on admin_init not init
Fix settings label
Improve front-end JS
Added styles settings tab
Added _halt_deprecated_function
Added widgets wrapper Halt_Widget class
Updated Sass stylesheets, removed Compass dependency
Removed autoload for classes, as well as shortcode wrapper

Going to use one single file for all shortcodes to keep it centralised
and clear.
Added few more allowed mime types
Was using 'stag' instead of 'halt' at many places

I must be high :/
Add Halt settings import notice
Minor edits, added Welcome screen badge