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
Fix code path regression

To make sure that sub_dirs and deps are properly added to the code path
regardless of -r/--recursive, add 'preprocess' and 'postprocess' to the
list of recursive commands. Without this fix, you'd have to invoke
'rebar -r ct skip_deps=true', if you wanted to run base_dir's ct suites
with deps and sub_dirs on the code path (while skipping all non-base_dir
ct suites).

So, with this patch applied, if you run
$ rebar ct
deps and sub_dirs will be on the code path, and only base_dir's ct
suites will be tested.

If you want to test ct suites in base_dir and sub_dirs, you have to run
$ rebar -r ct skip_deps=true

If you want to test ct suites in all dirs, you have to run
$ rebar -r ct