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/Cache.py

@@ -0,0 +1,40 @@
+from __future__ import absolute_import, division, print_function, unicode_literals
+
+from collections import defaultdict
+
+class Cache(object):
+    def __init__(self):
+        self._value_to_index = {}
+        self._index_to_value = []
+
+    def value_to_index(self, value, **kwds):
+        index = self._value_to_index.get(value, None)
+        if index is None:
+            index = len(self._index_to_value)
+            self._index_to_value.append((value, kwds))
+            self._value_to_index[value] = index
+        return index
+
+    def index_to_value(self, index):
+        return self._index_to_value[index]
+
+def NamedCache():
+    return defaultdict(Cache)
+
+def cache_by_key(d, keyfunc=None, exclude=None):
+    cache = defaultdict(Cache)
+    exclude = exclude or None
+    keyfunc = keyfunc or (lambda x: x)
+
+    def visit(item):
+        if isinstance(item, list):
+            for i, x in enumerate(item):
+                item[i] = visit(x)
+
+        elif isinstance(item, dict):
+            for k, v in item.items():
+                item[k] = visit(v)
+
+        return item
+
+    return cache