RSS reader/manager restructuring

db6ea86 should actually have been part of the settings cleanup PR I merged yesterday but I must have missed it. When applying the commit to current master I took the time to do some cosmetics/cleanup work and also moved IRssObserver and CRssManager into seperate header/source files and turn CRssManager (which is currently a global) into a singleton.
Functionality and behaviour haven't been touched, it's all just restructuring and cleanup.

Xcode changes not included.