Refactor OKComputer routes into an engine, and isolate its namespace

I've refactored to better use Rails' engine related features. Previously, OKComputer was mounting mounting its routes directly into Rails. It's nicer if we mount it instead as an engine (see change in `/config/routes.rb`). The engine is now automatically mounted at `/okcomputer`, and you can change this via a new `OkComputer.mount_at` option. (You can also set `mount_at = false`, which disables automatic mounting)

You'll now see this in `rake routes`:

```
okcomputer  /okcomputer             OKComputer::Engine

Routes for OKComputer::Engine:
root GET|OPTIONS /                 ok_computer/ok_computer#show {:check=>"default"}
 all GET|OPTIONS /all(.:format)    ok_computer/ok_computer#index
     GET|OPTIONS /:check(.:format) ok_computer/ok_computer#show
```

**Caveat**

It's a best practice to use `isolate_namespace` on engines (see http://guides.rubyonrails.org/engines.html), which means you namespace your controllers. However for some reason I don't understand, Rails doesn't seem to like the namespace `OKComputer` possibly because of the 3 consecutive capital letters, and I've had to add the following:

```ruby
# in lib/okcomputer.rb
OkComputer = OKComputer

# in spec/spec_helper.rb -- normally this class is loaded via autoload...
require OKComputer::Engine.root.join("app/controllers/o_k_computer/ok_computer_controller")
```

Alternatives to this are:

1) Change the gem's namespace to `OkComputer` with a lowercase 'k' (magically fixes above issue). You can see this at commit https://github.com/johnnyshields/okcomputer/commit/93643bb5e4d0461106d906e9e45d549bc4fc41fb. We could add `OKComputer = OkComputer` for backward compatibility of configs (or make it a shim which throws deprecation warnings to users)

2) Remove `isolate_namespace`