block/merkleblock support in framer.js. one other change and fix.

This adds packet creation for `block` and `merkleblock` (as well as `tx` which was already their for reading from storage). Maybe soon we can implement sending these packets.

I also made two changes: I fixed an implicit global var, and I added a `subtype` property for the block object to keep track of whether it is a `block` or a `merkleblock`. Another small fix pertaining to ensuring `hashes` and `flags`.