Fixes #826; Redirect info messages to *err*

I decided to leave some messages alone. Specifically

 * messages which look like stacktraces
 * messages which instruct the user to take some sort of action

Arguably the stacktraces should go to stderr but I thought I'd leave
those open for discussion. Any others that remain represent errors on
my part.
Use warn instead of info where applicable
Pick up a 'WARNING' that got missed
Merge remote-tracking branch 'cddr/826-info-messages-to-stder'

Conflicts:
	leiningen-core/src/leiningen/core/classpath.clj
Silence warn calls when LEIN_SILENT is set.