Allow Browsers to be a single array

Allow the user to specify a single array to the config, rather than an array of arrays

Currently you need to do something like:

```ruby
Sauce.config do |c|
  c[:browsers] = [
    ["Windows 8", "Internet Explorer", "10"]
  ]
end
```

With this change you can do:

```ruby
Sauce.config do |c|
  c[:browsers] = ["Windows 8", "Internet Explorer", "10"]
end
```

Also, since there is logic in the setter, I've changed `method_missing` to call it, rather than accessing `@opts` directly.