Reorganize visitor/mapper code

[JonoYang]

Currently we are putting code for the on-demand/priority queue in with the visitor code. It would be better if we separated the code. We should also decide which term we want to use for the on-demand/priority queue, since we use both currently. It is also worth considering if we want to merge the visitor and mapper code as well.

[pombredanne]

See also #521

[JonoYang]

The visitors and mappers have been merged into a single module named miners.