Add a class StreamAction to manage the effect of a stream on an entity.
Update comment.
Remove useless field.
Reimplementing streams not as a hero state.

This will let the hero to do other actions while in a stream (if the
stream allows them): use the sword, use an item or move.
Streams: block the control from the player if needed.
Reimplementing streams without hero state.
Fix movement end of north streams.
Stop blocking streams if there is an obstacle after them.
Fix detecting if a stream can be started due to obstacles.
Fix the speed of diagonal streams.
Fix diagonal streams ending up on adjacent streams.
Add a TODO for bombs on streams.
Constness.
Don't allow to run on blocking streams.
Update changelog about streams.
Talk about stream collision rule in ChangeLog.
Fix non-blocking streams. Streams are now done. Fix #288.
Merge branch 'stream'
Fix streams leading to obstacles by centering the hero on them. Fix #479
Add methods stream:get/set_direction().