coala_html.py: Fix bug in copying angular package

The current approach depends on `os.walk` to get the content
for a directory and then determines if the directory is empty.
The current approach is bugy in implementation and also the
approach isn't the best approach as it requires traversing
the directories recursively just to make sure that parent
directory is not empty.

The new approach with this commit, does the job by checking
for the existence of `index.html` in target directory. Since
we already know the structure of target directory which happens
to be a known angular package, we can just check if the `index.html`
exists in the target directory.

Fixes #83

coalahtml/coala_html.py

@@ -35,7 +35,7 @@ def main():
 
     dir_path = create_dir(os.path.abspath(args.dir))
 
-    if len(list(os.walk(dir_path))) > 0:  # pragma: no cover
+    if not os.path.isfile(os.path.join(dir_path, 'index.html')):
         copy_files(get_file(Constants.COALA_HTML_BASE), dir_path)
 
     if not args.noupdate: