Expose a new Serialize function on Signature type.

This commit exposes a new function named Serialize on the Signature type
which can be used to obtain a DER encoded signature.  Previously this
function was named sigDer and was part of btcscript, but as @donovanhide
pointed out in issue btcscript/#3, it really should have been part of this
package.

ok @owainga
Add tests for new Signature.Serialize function.

This is part of the resolution for issue btcscript/#3.