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.
I've done a lot of cross 2.7/3.x stuff before.  If any inconsistencies jump out at you, let me know, but my plan is to get it mature and THEN test it in 3.x - my theory is that it'll take me 30 minutes to fix at the end, which saves time over "testing it twice every time" (which isn't trivial because getting ledgers is very slow and even reading them from disk is slow because they're so large)...