Add postfix return statements

They work like this:

    readFiles = (directory, cb) ->
      fs.readdir directory, (err, files) ->
        cb err and return if err?
        output = {}
        for file in files then do (file) ->
          fs.readFile path.join(directory, file), 'utf8', (err, data) ->
            cb err and return if err?
            output[file] = data
            files.pop()
            cb undefined, output if files.length is 0

`and return`, appended to a statement, parses to a POST_RETURN token,
which triggers a special case on the Return node. The token is generated
through the rewriter, by watching for and tagging any combination of
`LOGIC/&&` followed by `RETURN/return`.