Add Amount.MulF64.

ok @davecgh, @jcvernaleo
Update test coverage report.
Switch bytes.Buffer to Reader where possible.

bytes.Reader is a little bit more efficient than a bytes.Buffer when
just reading, so in situations where only an io.Reader is needed (for
Block and Tx deserialization), switch to a bytes.Reader.

ok @davecgh
Add NewBlockFromReader and NewTxFromReader.

While here, remove the serializedTx field from Tx.  This field was
originally intended to be used to cache the bytes of the serialized
transaction, but it was never used and can effectively leak memory if
the Tx was created with a call to NewTxFromBytes.

ok @davecgh
Update test coverage report.
IMPORT PATH MODIFICATION CHANGESET
----------------------------------

Please use this changeset as the first one on top of the conformal codebase.
Note that if there are new imports, they need to be changed as part of this
changeset. In general, use "git rebase" on github.com/conformal's version
and apply this changeset first.