Add option to link files instead of moving or copying them.

Hi,
I've made modifications to beets so that it links in the files instead of copying or moving them. Beets now links when both link and copy are set to no in the yaml since I haven't been able to get a link: yes option into the configure['import'] map.

So this works, as far as I can tell. But I would need help with what I should add where to have for command line flags and config file settings. It only works on systems where Python implements link i.e. not windows.