Add back account APIs in compiler

[spencercorwin]

I removed all the Account APIs because I thought accounts were no longer a thing in N3. But you can still call GetAccountState on the NEOToken native contract, which returns an account state, which has a balance (of NEO), and two other properties which we can provide as easy APIs. The main important one is to be able to get the NEO (and GAS) balances of an address.

[spencercorwin]

Maybe want to just put a getBalance static method on the Address class? Or on the Account class?