Expose new SerializeSize API for blocks.

This commit adds a new function named SerializeSize to the public API for
MsgBlock which can be used to determine how many bytes the serialized data would
take without having to actually serialize it.  In addition, it makes the
exported BlockVersion an untyped constant as well as changes the block and
tx versions to a signed integer to more closely match the protocol.

Finally, this commit also adds tests for the new function.

The following benchmark shows the difference between using the new
function to get the serialize size for a typical block and serializing
into a temporary buffer and taking the length of it:

Bufffer: BenchmarkBlockSerializeSizeBuffer     200000          27050 ns/op
New:     BenchmarkBlockSerializeSizeNew     100000000             34 ns/op

Closes #19.