Removed $allowed_panels, replaced with excluded_panels config() setting.

Removed $allowed_panels static & setter.

Replaced with the ability to define excluded panels in YAML config. eg.:

**/mysite/_config/dashboard.yml**
    
    Dashboard:
        excluded_panels:
            - 'DashboardGoogleAnalyticsPanel'
            - 'DashboardQuickLinksPanel'
Merge pull request #36 from purplespider/patch-2

Removed $allowed_panels, replaced with excluded_panels config() setting.