dx download glob support

@kjlai @psung 

Adds support for globs in the last elements of paths, e.g.

```
dx download foo*
dx download *.fast?
dx download -r /foo/bar*/
dx download -r -f project1:/foo*/ project2:/bar* --output /local/dir/
```
And various other forms of that.

Makes it a non-error to try to download non-file objects and open files (prints a warning instead). Haven't yet decided if this needs a different exit code or any way to control whether this should be an error.

Adds `--all` for getting all objects with the same name, which is not yet useful because it doesn't rename them and instead tries to overwrite one with another and errors out.

Does not yet support full shell-style path element globbing. For example, `dx download /foo*/*.bam` won't work as expected.

Needs a test case, but until I write one, please hammer on this when you have some time.