Add PayToPubKeyHashScript and SignatureScript functions.

PayToPubKeyHashScript generates a new pay to pubkey hash script to use
as the pkScript when creating new transactions.  If the passed pubkey
hash is an invalid size, StackErrInvalidOpcode will be returned as an
error.

SignatureScript returns the signature script necessary to validate a
single input of a transaction.

This also adds sanity checking for serializing scripts into byte
slices.  If the length of a serialized opcode does not equal the
expected length, StackErrInvalidOpcode will be returned when unparsing
a []parsedOpcode.

New internal tests were added to verify checks for valid and invalid
parsed opcodes.