More improvements to exchange_rate plugin

- better historical rate handling, including caching
- grabbing and scanning wallet transactions no longer needed
- fix autosize of fiat column
- more efficient

lib/util.py

@@ -7,6 +7,7 @@ import traceback
 import urlparse
 import urllib
 import threading
+from i18n import _
 
 def normalize_version(v):
     return [int(x) for x in re.sub(r'(\.0+)*$','', v).split(".")]
@@ -15,7 +16,6 @@ class NotEnoughFunds(Exception): pass
 
 class InvalidPassword(Exception):
     def __str__(self):
-        from i18n import _
         return _("Incorrect password")
 
 class MyEncoder(json.JSONEncoder):
@@ -182,13 +182,15 @@ def format_satoshis(x, is_diff=False, num_zeros = 0, decimal_point = 8, whitespa
         result = " " * (15 - len(result)) + result
     return result.decode('utf8')
 
-def format_time(timestamp):
-    import datetime
+def timestamp_to_datetime(timestamp):
     try:
-        time_str = datetime.datetime.fromtimestamp(timestamp).isoformat(' ')[:-3]
+        return datetime.fromtimestamp(timestamp)
     except:
-        time_str = "unknown"
-    return time_str
+        return None
+
+def format_time(timestamp):
+    date = timestamp_to_datetime(timestamp)
+    return date.isoformat(' ')[:-3] if date else _("Unknown")
 
 
 # Takes a timestamp and returns a string with the approximation of the age