Copying directories with DNAnexus paths includes all of source path

[jtratner]

When copying directories, the local directory arguments specified in CLI or in copytree should be stripped off of target path. Somehow this is not working correctly in current stor. It needs to be fixed.

Current behavior:

stor cp -r path/to/localdir/ dx://project:/remotedir
stor list dx://project:/remotedir
dx://project:/remotedir/path/to/localdir/file1

Expected behavior (as with swift and local filesystem):

stor cp -r path/to/localdir/ dx://project:/remotedir
stor list dx://project:/remotedir
dx://project:/remotedir/file1