add test suite

I expect that it return boolean. Also in ruby is convention to have such method as `preset?` instead of java style `is_preset`
so it can look like
```
def self.preset?(key)
  PRESETS.has_key?(key)
end
```
see http://www.ruby-doc.org/core-2.1.2/Hash.html#method-i-key-3F

another idea. I think there is mixture of presets and fonts. I think presets deserve own class which can have such method like `PreSet.new(key)` or `PreSet.available?(key)` This way you also remove quite annoying switching of internal variables and can have only simple `@preset` and if you need to have accessor you can use delegator - http://www.ruby-doc.org/stdlib-2.0/libdoc/forwardable/rdoc/Forwardable.html