Collect, don't clobber, results in BatchLookupResolver.initialize()

Previously, `_lookup()` returned a dict like `{names:
[addresses]}`. `initialize()` iterated through matching results, calling
`update` on the `lookup_cache` dict for each match. Each update call
would clobber any existing data in a `name` key with a new set of
`addresses`. Practically, this meant that queries that should match
several instances and therefore return round-robin records (with all
matching addresses) instead returned plain A records (with the last
matching address).

Here, we add another loop through the `_lookup()` result items and
extend the cache dict with all new addresses found. This restores the
expected round-robin behavior.
Merge pull request #10 from SimpleFinance/fix-id-lookup

Collect, don't clobber, results in BatchLookupResolver.initialize()