add Emacs/vi header to non-dummy test modules
Merge pull request #400 from vladdu/headers

add Emacs/vi header to non-dummy test modules
Consistently format export attributes
Cache code paths to reduce filename:absname/1 calls

rebar_utils:expand_code_paths is called enough times that the amount of
code path entries to be transformed to absolute paths leads to an
abundant number of filename:absname/1 calls. To fix that, cache
previously converted paths in a set.