Add autoload to ess-help.el

Since the last update (20131027.2006), calling help on an object in the iESS buffer opened the help text in the same buffer, changing its status to 'read-only'. Adding this line to ess-help.el fixes this.
Merge pull request #67 from plmtznr/patch-1

autoload ess-with-current-buffer in ess-help.el