Add a print-method for Operation to avoid blocking

When an operation is printed at the repl, it shouldn't cause the repl block.
Use three value returns for failed? and complete?

Return nil if the operation is still running.
Add a running? predicate to the Control protocol

The predicate returns true if the operation is still running, and has not
completed or failed.
Updated project.clj, release notes and readme for 0.2.1
Merge branch 'release/0.2.1'
Merge tag '0.2.1' into develop

pallet-fsmop-0.2.1
Updated version for next release cycle