kiji-express: EXP-246. provide implicit conversions of KijiPipe->Pipe and KijiPipe->RichPipe

issue: https://jira.kiji.org/browse/EXP-246
review: https://review.kiji.org/r/1246
commit: https://github.com/kijiproject/kiji-express/commit/d3cb40d702058f866e98d69ddc620cf407f780df