kiji-express: EXP-109. Create a Maven archetype for a skeleton KijiExpress “flow” project.

issue: https://jira.kiji.org/browse/EXP-109
review: https://review.kiji.org/r/1490
commit: https://github.com/kijiproject/kiji-express/commit/d81af1befea0a6d3bab7f4d0da0488041cee8bf8