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: