Rework directory structure to follow go standards

http://golang.org/doc/code.html describes how code should be structured.
Make BigIntPoly a private type (bigIntPoly)
Privatize all functions in arith.go (except for FloorRoot())
Avoid use of aks.FloorRoot() in aks-go/bin-src/aks.go and privatize it
Make aks.GetAKSWitness() take a start and end parameter
Add a -start flag for the lower bound to use