Build folly with bazel

[Ming Zhao]

After reading the mailing list and complicated instructions on how to
build folly, I think it would be great to build folly with bazel, and
here it is:

https://github.com/mzhaom/trunk/blob/master/third_party/folly/BUILD

Other than a few pull requests(like
https://github.com/floitsch/double-conversion/pull/12) and some bug
report I'm waiting for someone to fix, it pretty much works out for
box for most stuff you intend to use from folly.

But definitely it's not fully completed yet, for example, codes under
wangle are not built yet. But I hope they could get addressed soon,
and welcome pull requests.

Thanks,
Ming