rebar_ct: fix Dialyzer warning caused by bcc57e9b

Also fix a single line exceeding max column.
rebar_core: fix consistency issues caused by 252b31f

- refactor plugin dirs code to be simpler and easier to read
- use erlang-mode's default (%%) comments for portability/consistency
- make sure erlang-mode's indenter is used so that a future whole
  buffer indent doesn't get messed up
Update THANKS file
Delete dialyzer_warnings on 'make distclean'