Add Serialize/Deserialize for MsgBlock and MsgTx.

This commit introduces two new functions for MsgBlock and MsgTx named
Serialize and Deserialize.  The functions provide a stable mechanism for
serializing and deserializing blocks and transactions to and from disk
without having to worry about the protocol version.  Instead these
functions use the Version fields in the blocks and transactions.

These new functions differ from BtcEncode and BtcDecode in that the latter
functions are intended to encode/decode blocks and transaction from the
wire which technically can differ depending on the protocol version and
don't even really need to use the same format as the stored data.

Currently, there is no difference between the two, and due to how
intertwined they are in the reference implementaiton, they may not ever
diverge, but there is a difference and the goal for btcwire is to provide
a stable API that is flexible enough to deal with encoding changes.