Improve interactive evaluation with special buffers

- Use "(ns $repl-ns)" in dummy file when ns form not found
- Use (buffer-name) as file name in buffers not visiting a file
- Take into account buffer names in cider-find-file (useful for buffers not
  visiting a file)
Strip :reload and :reload from ns-form
Make `cider-repl-set-ns` to ask for ns in buffers with no ns form
Merge pull request #815 from vitoshka/scratch-ns

Use "(ns $repl-ns)" in dummy file when ns form not found