Make NSObject category optional

[Ashton-W]

I prefer not to use NSObject categories in my projects. KVOController includes one in it's header file now, can we move it into another header file so it is opt-in? Maybe even a subspec in the podfile?

[grp]

That was added recently from a pull request: 625cfe9 All the methods are prefixed with the name of the project (KVOController), for what it's worth.

I'm not sure if we'll be able to make the category optional soon, but it's a valid request and a pull request would be great!