Add SiaRandomNumber

Quick fix on the Makefile, entropy_test.go reduced to 1 function, struct
State removed and all variables added to struct Blockchain. Every piece
of the entropy process should be complete at this point.
Resolved merge conflict
Merge branch 'master' of github.com:Siacoin/Sia
Merge pull request #11 from DavidVorick/master

Add SiaRandomNumber