remove dependency on wget for 'build' and 'verify'

To make the core tools portable to platforms like Mac OS X and Windows,
remove the dependency on wget and instead use Python Requests, which
probably has better performance anyway.

fdroidserver/common.py

@@ -22,6 +22,7 @@ import sys
 import re
 import shutil
 import glob
+import requests
 import stat
 import subprocess
 import time
@@ -2070,3 +2071,17 @@ def string_is_integer(string):
         return True
     except ValueError:
         return False
+
+
+def download_file(url, local_filename=None, dldir='tmp'):
+    filename = url.split('/')[-1]
+    if local_filename is None:
+        local_filename = os.path.join(dldir, filename)
+    # the stream=True parameter keeps memory usage low
+    r = requests.get(url, stream=True)
+    with open(local_filename, 'wb') as f:
+        for chunk in r.iter_content(chunk_size=1024):
+            if chunk:  # filter out keep-alive new chunks
+                f.write(chunk)
+                f.flush()
+    return local_filename