Code review comment for lp://staging/~thumper/launchpad/scanner-awesomeness

I pointed out a couple of typos on IRC.

I think a test of CachedIterator that iterated two iterators in parallel like so:

ci = CachedIterator([...])
i1 = iter(ci)
assert i1.next() == ...
i2 = iter(ci)
assert i2.next() == ...
assert i1.next() == ...

I think it'll pass fine, but it's something my devious mind wants to see tested.

CachingIterator needs some blank lines between its members.

Otherwise it all looks nice, and will be a nice change!