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.
add Emacs/vi header to non-dummy test modules
Update reference to installation notes
Merge pull request #403 from sdepold/patch-1

Update reference to installation notes
Merge pull request #400 from vladdu/headers

add Emacs/vi header to non-dummy test modules
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
Add {prefix, atom()} option for abnfc_opts

Specify the atom to use as a prefix for the decoder and generator
functions inside the parser's module.  Default is ''.
Add {parser, module()} option for abnfc_opts

Specify the module to use for parsing the ABNF grammars.  Default is
'abnfc_rfc4234'.
Add {qc, proplist()} option for abnfc_opts

Specify the options to use for the generator functions inside the
parser's module.  Default is [].