Remove implicit queueing

There are 2 cases where commands are being queued: when using `#combine_options` and when using `#method_missing` methods (like `#resize`). This commit drops the `#method_missing` queueing.

The reason for this is that the code becomes much more complicated when at the beginning of each "terminal" method we have to run the queue if there are any queued commands. But when we write `image.resize("150x150")`, we don't expect it to be queued. I can imagine a user writing the following code, and being suprised it isn't working:

```ruby
path = "path/to/image.jpg"
image = MiniMagick::Image.new(path)
image.resize("150x150")

# User: "Done! The image is cropped."
```

We already have a great API for queueing commands -- `#combine_options`, which I'm sure users have already adopted, and that no-one will be suprised if the regular commands aren't queueing anymore.

I will thoroughly update the README before the next release.

Fixes #230.