Added a masterpage

Hey, 

i have added a master page that contains all basic css & scripts and so on. 
Every page now uses this page and it is now even easier to create plugin pages. 

To create a page now you have to do: 
```jade
extends /layout

block declarations
   - additional_title = 'Page Title';
   - use_knockout = true|false;

block additional_css
    link(href="/link/to/my/css")

block additional_scripts
    script(src="/link/to/my/js")

block body
    h1 This is the content
````

I also added support to use MediacenterJS as a Web-App on Homescreen on iOS.