Test pulling remote files using ADD in a buildfile.
Buildfile: for ADD command, determine filename from URL.

This is used if the destination is a directory. This makes the URL
download behavior more closely match file copying.

Fixes #1142.
Remove some trailing whitespace.
Update ADD documentation to specify new behavior.
Merge pull request #1158 from cespare/1142-docker-add-fix

*Buildfile: determine a filename from a URL if the destination is a directory