Batch Tree

Add a 'batch tree' structure in 'batchtree.go'.
Removed all of the old disk files, as updates to the spec have obsoleted the code.

You can insert into a batchTree, delete from the batchTree, and select a sector at random from the batchTree. The most difficult building blocks are now in place for performing proof-of-storage.

I am waiting to add these changes to the quorum code because I know this will result in massive merge conflicts with Luke's code.