Fix 'rebar generate' regression (#253)

If the directory we're about to process contains
reltool.config[.script] and the command to be applied is
'generate', then it's safe to process. We do this to retain the
behavior of specifying {sub_dirs, ["rel"]} and have "rebar generate"
pick up rel/reltool.config[.script]. Without this workaround you'd
have to run "rebar -r generate" (which you don't want to do if you
have deps or other sub_dirs) or "cd rel && rebar generate".
Merge pull request #254 from tuncer/fix-generate

Fix 'rebar generate' regression (#253)
Fix 'rebar help clean' function_clause error

Missing info(help, clean) in rebar_qc.erl.
Merge pull request #265 from goofansu/fix-qc-help-clean

Fix 'rebar help clean' function_clause error