Shouldn't mix static and instance methods in TokenContract and subclasses

[devhawk]

TokenContract defines abstract instance methods Symbol and Decimals, but the other methods like TotalSupply, BalanceOf as well as subclass methods such as Transfer and OwnerOf are static. It seems odd from a C# semantics perspective to mix and match like this

[erikzhang]

Symbol and Decimals must be instance methods because they are abstract. The others are static because static methods are cheaper.