Consider deleting generated resource directories on build.

[DavidSouther]

In grunt-docco, users were noticing sometimes the build directories were incorrect. https://github.com/DavidSouther/grunt-docco/issues/40

This was traced to node-fs-extra and ncp. https://github.com/jprichardson/node-fs-extra/issues/98 https://github.com/AvianFlu/ncp/pull/75

While the behavior should be fixed in ncp itself, it might be a good idea to consider having docco remove the resource directory before copy.

[jashkenas]

Ugh!