`api` Package for contract

[kfangw]

• Define functions and wrapper functions for contract execution

API	Desc	Note
PutStorage(key, value) -> bool,err	Write k/v to contract's own storage	int,string are support
GetStorage(key) -> value,bool	Read value of a key from contract's own storage
WithdrawBalance(to,balance) -> bool	Withdraw this contract's balance to another address
GetBalance() -> int	Get this contract's balance to another address
CallContract(ContractAddress, method, args) -> value,err	Inter contract Call
Now() -> int	return timestamp in seconds
BlockHeight() -> int	return height of this block
BlockPrevHash() -> string	return hash of prev block
tx.Source -> string(address)	return source of this tx
log.Print	loging?