EXP-19. Adds Extractor trait for Model SPI.

Reviewed by: kiyan
EXP-21. Adds Scorer trait for Model SPI.

Reviewed by: kiyan
Update release notes for 0.3.0 release.

Reviewed by: kiyan
[maven-release-plugin] prepare release kiji-express-0.3.0
[maven-release-plugin] prepare for next development iteration