Extract ActionView to separate directory

Hey,

This is continuation of work done by @drogus. Basically it moves ActionView outside of ActionPack to its own directory. AV still remains dependency on AP, I will be working on this in coming weeks.

However, after discussing it with @drogus, both of us agreed that it would be good to have those commits in master. (they are basically just moving files, not much changes is code itself) because:

* @kaspth is working on his GSoC Loofah project - it touches ActionView so it will be super hard to solve conflicts either for him or me once one of our branches get merged.
* It'll be hard for me if somebody will make some changes in master to rebase it.

/cc @pixeltrix @drogus @kaspth