New command line LedgerTool. (RIPD-243)

* Retrieve and process summary or full ledgers.
* Search using arbitrary criteria (any Python function).
* Search using arbitrary formats (any Python function).
* Caches ledgers as .gz files to avoid repeated server requests.
* Handles ledger numbers, ranges, and special names like validated or closed.

bin/ripple/util/Time.py

@@ -0,0 +1,21 @@
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+import datetime
+
+# Format for human-readable dates in rippled
+_DATE_FORMAT = '%Y-%b-%d'
+_TIME_FORMAT = '%H:%M:%S'
+_DATETIME_FORMAT = '%s %s' % (_DATE_FORMAT, _TIME_FORMAT)
+
+_FORMATS = _DATE_FORMAT, _TIME_FORMAT, _DATETIME_FORMAT
+
+def parse_datetime(desc):
+    for fmt in _FORMATS:
+        try:
+            return datetime.date.strptime(desc, fmt)
+        except:
+            pass
+    raise ValueError("Can't understand date '%s'." % date)
+
+def format_datetime(dt):
+    return dt.strftime(_DATETIME_FORMAT)