remove explicit class registration, minor whitespace fixes

@karlitschek sorry to mess with your code. I removed the explicit class registration and renamed the class files in lib accordingly so they are automagically found by the classloader. I also renamed the Hook class to Hooks, because it contains code for more than one hook. I also removed the unnecessary namespace from classes where applicable and used `self::` instead of `HOOK::` to access static variables. There were also a few whitespaces missing and I found a `?>` that neede to be killed with fire. All of this only took about 25 min.

I want the activities app to be a great success, so I just HAD to polish this a bit. People will have a look at it to learn how things are done in owncloud apps. There are also a lot of lines exceeding our 80 columns limit .... maybe someone could reformat those extraneous method paramters ...

@DeepDiver1975 @karlitschek @kabum @bantu please review.