app.net: Support for inline links, new temporary paths functions, privacy_image_cache now with different sizes

This pull request mainly consists of changes for the appnet connector. it now supports links in markdown style and has a better error logging. The only thing that is missing is the queue mechanism for failed posts.

The minor changes:
* tumblr: the api key values can now be changed via the admin page
* Twitter: If there is an embedded picture and a link, then the picture is taken for the link details. Before that it happened, that there were two pictures in the post and often they were the same
* buffer: It wasn't possible to save the admin settings
* privacy_image_cache: It is now possible to use parameters like ":medium" or ":large" behind the cache urls. (Just like Twitter does)
* posterous and unhosted-enabled storage: They are now declared as unsupported