Implement enough of chunk.IncompleteHashTree to be usable.
Rearrange download: all block/hash requests now go through
a ValidatedBucket instance, which is responsible for retrieving
and verifying hashes before providing validated data. Download
was changed to use ValidatedBuckets everywhere instead of
unwrapped RIBucketReader references.
Note that using "whatever version of python the name 'python' maps to in the current shell environment" is more error-prone that specifying which python you mean, such as by executing "/usr/bin/python setup.py" instead of executing "./setup.py". When you build tahoe (by running "make") it will make a copy of bin/allmydata-tahoe in instdir/bin/allmydata-tahoe with the shebang line rewritten to execute the specific version of python that was used when building instead of to execute "/usr/bin/env python".
However, it seems better that the default for lazy people be "whatever 'python' means currently" instead of "whatever 'python' meant to the manufacturer of your operating system".
