Merge pull request #928 from bholt/repl+execution

Use Execution to run flows in REPL
Clean up ExecutionContext a bit
Use executionContext.buildFlow in Job
Fix Issue 932: no-op Jobs should not throw
Merge branch 'develop' into execution-cleaning
Merge ExecutionContext changes with REPL
Merge pull request #934 from twitter/fix-932

Fix Issue 932: no-op Jobs should not throw
Merge pull request #933 from twitter/execution-cleaning

Clean up ExecutionContext a bit
merge develop (includes Execution improvements)