Prior to this commit, Vagrant would attempt to path expand a file that
didn't exist if it was left out of the passed in arguments and no
`--url` was used for external box uploading. This commit fixes that by
adding some additional validation for the passed in box file.
This commit introduces a new uploader class for uploading files and
splits up some commonly used functionality between it and the downloader
class into a curl helper library.
Brian Cain