Refactor and fix `FixCamera#follow()`

So sorry I procrastinated this, I finally sat down and read through how `FlxCamera` handles following targets (and it was much more simple than I thought it would be).

This pull request fixes [the last tiny error with `FlxCamera`](https://github.com/FlixelCommunity/flixel/pull/189#issuecomment-28607960) (and I now understand why that `*0.25` part was accidentally included), but also is tidies up `FlxCamera#follow()` by merging similar camera styles and renaming variable names to be more descriptive.

I have tested and confirmed that it works as expected, at least in [Mode](https://github.com/FlixelCommunity/Mode). If anyone else wants to test the resulting SWC:
* https://github.com/FlixelCommunity/flixel/releases/download/untagged-e6aefb20eafd21679aca/flixel-v2.55-183-g692a614.swc