rose.stem: fix `rose` global name.

N.B. `rose` was implicitly imported by `import rose.run` before #741.
Merge pull request #745 from matthewrmshin/fix.rose-stem.import

rose.stem: fix `rose` global name.
rose.suite_run: fix bad variable name.

Bug introduced in #741 by a rename.
Merge pull request #746 from matthewrmshin/fix.rose.suite_run

rose.suite_run: fix bad variable name.
Update CHANGES.md
Update README.md
rose.fs_util: improve symlink logic.

Add filename to OSError if target exists.
Don't use os.path.exists for testing existence of symlink. It does not
work on broken link.
Merge pull request #747 from matthewrmshin/symlink.e.filename

rose.fs_util: improve symlink logic.