database: switching from mysql to sqlite

Currently we only need the db to keep track of the files that were
already downloaded, thus we only use one database and only one table.
A complete sql database is a bit too bulky for this purpose.
By using sqlite we can minimize and embed the db.

src/studip.py

@@ -146,13 +146,10 @@ class Studip:
         last_dl = self.db.get_last_file_dl(doc)
         if last_dl == None or last_dl < doc_chdate:
             rsp2 = self.auth_req('/api.php/file/' + doc + '/download')
-            #total_size = int(rsp2.headers.get('content-length', 0))
             log.info('downloading ' + doc_name)
-            #progbar = tqdm(total=total_size, unit='iB', unit_scale=True)
             try:
                 with open(doc_name, 'wb') as doc_file:
                     for chunk in rsp2.iter_content(self.CHUNK_SIZE):
-                        #progbar.update(len(chunk))
                         doc_file.write(chunk)
                 self.db.set_last_file_dl(str(doc), str(int(time.time())))
             except OSError:
@@ -172,7 +169,7 @@ class Studip:
         try:
             subdirs = rsp.json()['subfolders']
         except ValueError:
-            return res_docs
+            return res_subdirs
         for subdir in subdirs:
             try:
                 sub_id = subdir['id']