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.
(cherry picked from commit fc68d81)
Add version in es-plugin.properties
(cherry picked from commit e7f5897)
prepare release elasticsearch-analysis-stempel-2.0.0.RC2
prepare for next development iteration
Add test stempel analyzer in mapping
Old versions (for es < 1.0) go to 1.x branch
Add plugin release semi-automatic script

We want to be able to release versions in a faster way then we do by now.

Closes #20.
prepare release elasticsearch-analysis-stempel-2.0.0
prepare for next development iteration