add put() method to fs

Every runner has some way of uploading local files to the filesystem, but mrjob.fs.base.Filesystem doesn't provide a consistent interface (it's basically read-only).

The Spark runner (see #1940) actually needs this because it supports all the filesystems. We just need to be able to upload a local file to a remote URI (no recursive behavior necessary).

GCSFilesystem already has a put() method, so we should follow that interface.