improve build process, part of #481

fix oneJar code coverage issue (coverage info corrupts JAR), test built SlamEngine JAR, allow environment variable overrides of backend-config, modify travis to use local mongodb override, auto-import zips data set when in travis builds. note: publish script has not been updated and jenkins is no longer expected to work.
One environment variable per backend. e.g. for the `mongodb` backend, the env variable name would be, `SLAMDATA_MONGODB`.