Use 'urllib2' for retrieving mbed library

For some reason when using 'urllib' in my environment (which includes a
Zscaler network proxy) 'urllib.urlretrieve()' does not work ending up
with an error message "301 Moved Permanently".
Python library 'urllib2' doesn't have this issue but does not provide
method 'urlretrieve()', so a little workaround is required.

Author: betzw

mbed/mbed.py

@@ -29,7 +29,7 @@
 import errno
 from itertools import chain, izip, repeat
 from urlparse import urlparse
-import urllib
+import urllib2
 import zipfile
 import argparse
 
@@ -281,7 +281,10 @@ def fetch_rev(url, rev):
         try:
             if not os.path.exists(tmp_file):
                 action("Downloading mbed library build \"%s\" (might take a minute)" % rev)
-                urllib.urlretrieve(url, tmp_file)
+                outfd = open(tmp_file, 'wb')
+                inurl = urllib2.urlopen(url)
+                outfd.write(inurl.read())
+                outfd.close()
         except:
             if os.path.isfile(tmp_file):
                 os.remove(tmp_file)
@@ -2427,4 +2430,4 @@ def main():
 
 
 if __name__ == "__main__":
-    main()
+    main()