Remove BlockHeader.TxnCount field.

This commit removes the TxnCount field from the BlockHeader type and
updates the tests accordingly.  Note that this change does not affect the
actual wire protocol encoding in any way.

The reason the field has been removed is it really doesn't belong there
even though the wire protocol wiki entry on the official bitcoin wiki
implies it does.  The implication is an artifact from the way the
reference implementation serializes headers (MsgHeaders) messages.  It
includes the transaction count, which is naturally always 0 for headers,
along with every header.  However, in reality, a block header does not
include the transaction count.  This can be evidenced by looking at how a
block hash is calculated.  It is only up to and including the Nonce field
(a total of 80 bytes).

From an API standpoint, having the field as part of the BlockHeader type
results in several odd cases.

For example, the transaction count for MsgBlocks (the only place that
actually has a real transaction count since MsgHeaders does not) is
available by taking the len of the Transactions slice.  As such, having
the extra field in the BlockHeader is really a useless field that could
potentially get out of sync and cause the encode to fail.

Another example is related to deserializing a block header from the
database in order to serve it in response to a getheaders (MsgGetheaders)
request.  If a block header is assumed to have the transaction count as a
part of it, then derserializing a block header not only consumes more than
the 80 bytes that actually comprise the header as stated above, but you
then need to change the transaction count to 0 before sending the headers
(MsgHeaders) message.  So, not only are you reading and deserializing more
bytes than needed, but worse, you generally have to make a copy of it so
you can change the transaction count without busting cached headers.

This is part 1 of #13.
Remove deprecated InvType_* constants.

This closes #13.