Copy task is stalling on numerous files/directories

Hello :-)

Yesteday I deployed 15K files on a `gh-pages` branch and the copying took ages, eventually stalled and never finished.

[By looking at the code](https://github.com/tschaub/grunt-gh-pages/blob/master/tasks/gh-pages.js#L11-18), I guess using a promise for each directory then for each file bloated everything.

Just generating the tree is horribly slow (dozen of minutes) and once it succeeded, one file was copied every 5 minutes.

Do you have an idea in mind to deal with that?

Thanks :-)