Can not upgrade stempel plugin from elasticsearch 0.90.5

Due to fix [3790](https://github.com/elasticsearch/elasticsearch/issues/3790) in core, upgrading an analyzer provided as a plugin now fails.

See https://github.com/elasticsearch/elasticsearch/issues/4936 for details.

Issue is in elasticsearch core code but can be fixed in plugins by overloading `PreBuiltTokenFilterFactoryFactory` and `PreBuiltAnalyzerProviderFactory`.

Closes #18.