Refactored request specs.

I introduced a `stub_request` method to simplify the fixture loading and method expectations. I also moved the message expectations out of the subject block and into a before filter, since I believe expectations do not belong in a subject block.

This simplifies this
```ruby
subject do
  expect(request.class).to receive(:get).with(request.api_url("game/by-summoner/1/recent")).and_return load_fixture('game', GameRequest.api_version, 'get')

  request.recent 1
end
```

to this

```ruby
subject { request.recent(1) }

before(:each) { stub_request(request, 'game', 'game/by-summoner/1/recent') }
```

which I believe is more readable. It also has the added benefit of making changing the tests easier if you ever decide to use a different http library.

The http GET verb is hard-coded into `stub_request`, but the API is currently does not, and is unlikely to ever, use any other http verb.

I changed the name of the `getbyid-team` fixture to `get-team-by-id`, to be consistent with `get-summoner-by-name` and `get-champion-by-id`, etc. This also makes the above simplifications possible.

I also moved the spec helper methods previously defined in `spec_helper` into a module in `support/helpers`. The makes the `spec_helper` file less cluttered. The two files named `helpers` and `model_helpers` may be confusing; if you can think of any better names I'll change them.

There are a few other, mainly cosmetic/structural, changes that I made to the request specs.

If there is anything you don't agree with, I'd be happy to discuss/revert it :)