Rename account to label where appropriate

This change only updates strings and adds RPC aliases, but should simplify the
implementation of address labels in
https://github.com/bitcoin/bitcoin/pull/7729, by getting renaming out of the
way and letting it focus on semantics.

The difference between accounts and labels is that labels apply only to
addresses, while accounts apply to both addresses and transactions
(transactions have "from" and "to" accounts). The code associating accounts
with transactions is clumsy and unreliable so we would like get rid of it.

test/functional/wallet_import_rescan.py

@@ -78,7 +78,7 @@ class Variant(collections.namedtuple("Variant", "call data rescan prune")):
 
         if txid is not None:
             tx, = [tx for tx in txs if tx["txid"] == txid]
-            assert_equal(tx["account"], self.label)
+            assert_equal(tx["label"], self.label)
             assert_equal(tx["address"], self.address["address"])
             assert_equal(tx["amount"], amount)
             assert_equal(tx["category"], "receive")