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
Merge pull request #338 from vladdu/app_src_script

Processing .app.src.script expects a single value to be returned.