Fix requires.files.file(url) when url contains '&'

Hi!

Function `requires.files.file(url)` fails when the url contains the character `&`, as found e.g. in Amazon S3 download links, and many more places.

Taking a look to the source code, wget is invoked the following way:

    func('wget --progress=dot:mega %(url)s -O %(path)s' % locals())

The problem could be easily solved using double quotes around `%(url)s` and `%(path)s` in the following way:

    func('wget --progress=dot:mega "%(url)s" -O "%(path)s"' % locals())

Thanks!

Related issue: https://github.com/ronnix/fabtools/issues/261