Prevent errors on WPMU plugin activation

Set an empty array for default enabled menus
Allow empty snippet name or code
Updated list table class
Add global functions file
Refactor plugin files to use my personal best practices
Separated admin messages from admin views
Moved admin views, messages and help tabs to a separate directory
Update subpackage names
Moved CodeMirror loading script to separate file
More code refactoring
Updated to CodeMirror 3.13
Establish 1.7.2 as next version
Remove required attribute from snippet name input
While we're at it, let's remove the (Optional) labels

because everything is optional!
More code refactoring
Fixes on the import snippets page
Rework table creation method
Make sure that an imported XML file is valid
Add Travis CI and PHPUnit structure
Added starting year to copyright