refactored ex NOOP filter & specs into filter base specs

the NOOP filter is now a dynamically defined class into the `spec/filters/base_spec.rb` and the Plugin#lookup method has been adjusted to support dynamically defined plugins.
ah yes. `lookup` is a class method and `@logger` is not initialized. is it worth creating a class instance @logger for this? @jordansissel? I could add something like:

```ruby
private
def self.logger
  @logger ||= Cabin::Channel.get(LogStash)
end
```
and call 
logger.debug(...) in `lookup`. 
watdayathink?