fixed custom constant names
differentiate between no license key given and license not activated in messages. fixes #24
limit update check to once per 30 minutes. fixes #22
make sure classes are only loaded once
Merge remote-tracking branch 'origin/master' into refactor

Conflicts:
	class-license-manager.php
	class-plugin-license-manager.php
	class-plugin-update-manager.php
	class-theme-license-manager.php
	class-theme-update-manager.php
	class-update-manager.php