Fix detection of .app.src.script (Reported-by: Vlad Dumitrescu)

rebar_app_utils:is_app_dir/1 failed to search for .app.src.script when
looking for an app file in src/.

While at it, fix inconsistent module export list.
Fix detection and processing of .app.src.script

Loading .app, .app.src or .app.src.script returns now the value as per
the documentation (was wrapped in a list in some cases).

More places in the code detect .app.src.script and handle it.
add tests for processing .app.src{.script} files
Consistently format export attributes
Merge pull request #338 from vladdu/app_src_script

Processing .app.src.script expects a single value to be returned.
Merge pull request #404 from tuncer/consistent-export-attributes

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.