Added failsafes for retrieving remote apps on settings->apps page, in case categories or app data are not found

Added method getAppstoreApps to OC_App
Added minor documentation

respect coding style

respect coding style

remove files_imageviewer and files_texteditor from blacklist so they can be activated in a multi apps environment

show a label if an app is an internal app or a third party app from the app store.
show a preview image of the app if available

finish the application store feature so that users can download and install ownCloud apps from our website. there is still a small problem with write permissions. will look into this later

Created the following function:
 - checkLoggedIn
 - checkAdminUser
 - redirectToDefaultPage