Make FlattrID a hexadecimal value

This adds support for flattr "things" that are not direct flattr numeric ids,
but hexadecimal hashes representing web pages outside of flattr.

fdroidserver/metadata.py

@@ -154,9 +154,14 @@ class FieldValidator():
 valuetypes = {
     FieldValidator("Integer",
                    r'^[1-9][0-9]*$', None,
-                   ['FlattrID'],
+                   [],
                    ['vercode']),
 
+    FieldValidator("Hexadecimal",
+                   r'^[0-9a-f]+$', None,
+                   ['FlattrID'],
+                   []),
+
     FieldValidator("HTTP link",
                    r'^http[s]?://', None,
                    ["Web Site", "Source Code", "Issue Tracker", "Donate"], []),