Subprinters

Hi,

I'm working on a project with boost multi_index_containers, and I've been looking for a gdb printer for those. I couldn't find one, so I wrote an experimental one for myself. In the process, I changed your printers to the (newer) subprinter framework, which allows them to be listed, enabled, and disabled individually from within gdb. I'm sending this pull request in case you might find these useful. They are also "inspired" by libstdcxx.

The ordered multi_index support is in a separate branch not included in this request, but you can see it under my fork. Let me know if you think that could also be useful to the main project.

BTW, I'm not sure how these pull requests work, let me know if I didn't get it right.

Matei