Add support for nulldata standard scripts.
Add support for nulldata standard scripts.

This commit adds the new standard script type for empty, but data carrying
and provably prunable transaction scripts.

Closes #2.

ok @owainga

adiabat/btcd,Roasbeef/btcd,BitGo/prova,qchain/btcd,mathiasrw/btcd,Roasbeef/btcd,soapboxsys/ombfullnode,ltcsuite/ltcd,jrick/btcd,soapboxsys/ombfullnode,DanielKrawisz/btcd,runeaune/btcd,DanielKrawisz/btcd,Cleawing/btcd,sysbot/btcd,adiabat/btcd,CryptocurrencyCabal/htcd,jcmincke/dcrd,jimmysong/btcd,hectorj/btcd,jrick/dcrd,pkothbauer/btcd,cqswxj2013/btcd,danoctavian/btcd,davecgh/btcd,decred/dcrd,wallclockbuilder/btcd,mathiasrw/btcd,cjepson/btcd,Dirbaio/btcd,aakselrod/btcd,jrick/btcd,jrick/btcd,Stash-Crypto/btcd,cqswxj2013/btcd,jcmincke/dcrd,toddfries/btcd,Cleawing/btcd,sysbot/btcd,Dirbaio/btcd,davecgh/btcd,stevenroose/btcd,FactomProject/btcd,BitGo/prova,ppcsuite/ppcd-old,ltcsuite/ltcd,ppcsuite/ppcd,btcsuitereleases/btcd,dajohi/btcd,tb00/btcd,dajohi/btcd,1d4Nf6/btcd,toddfries/btcd,jongillham/btcd,pkothbauer/btcd,Stash-Crypto/btcd,dan-da/btcd,jongillham/btcd,ppcsuite/ppcd-old,hectorj/btcd,stevenroose/btcd,btcsuite/btcd,btcsuite/btcd,qchain/btcd,CryptocurrencyCabal/htcd,jimmysong/btcd,glv2/ppcd,runeaune/btcd,jrick/dcrd,wallclockbuilder/btcd,dajohi/btcd,glv2/ppcd,decred/dcrd,FactomProject/btcd,1d4Nf6/btcd,aakselrod/btcd,tb00/btcd,ppcsuite/ppcd,danoctavian/btcd,cjepson/btcd,Dirbaio/btcd,jcmincke/dcrd,dan-da/btcd,btcsuitereleases/btcd