RPC: mergecoins fix
RPC: mergecoins fix

* Move maturity verification to SelectCoinsSimple();
* Verify transaction sum correctly;
* Replace an intermediate transaction signing with estimation of transaction size for performance purposes.

penek/novacoin,elambert2014/novacoin,fsb4000/novacoin,gades/novacoin,penek/novacoin,fsb4000/novacoin,fsb4000/novacoin,gades/novacoin,stamhe/novacoin,gades/novacoin,penek/novacoin,novacoin-project/novacoin,penek/novacoin,byncoin-project/byncoin,elambert2014/novacoin,stamhe/novacoin,novacoin-project/novacoin,elambert2014/novacoin,gades/novacoin,elambert2014/novacoin,gades/novacoin,byncoin-project/byncoin,novacoin-project/novacoin,byncoin-project/byncoin,stamhe/novacoin,penek/novacoin,penek/novacoin,novacoin-project/novacoin,stamhe/novacoin,gades/novacoin,byncoin-project/byncoin,fsb4000/novacoin,elambert2014/novacoin,stamhe/novacoin,byncoin-project/byncoin,fsb4000/novacoin,novacoin-project/novacoin,fsb4000/novacoin,byncoin-project/byncoin,novacoin-project/novacoin,elambert2014/novacoin