Operation Type Implementation

## Background
We defined operation type for the transfer feature(`payment`, `createAccount`) until now.
And the methods which is defined in `OperationBody` interface are `Validate()`, `IsWellFormed()`, `TargetAddress()`, `GetAmount()`.
There is a need to define more operation type increasingly.
But the methods of `OperationBody` interface isn't general for other operation type.(e.g. `operation_voting_result`, `operation_proposal` ...). However `OperationBody` implementation for voting result should have GetAmount() method to satisfy interface.
It will be good if `OperationBody` struct doesn't need to implement unnecessary methods repeatedly and use current `OperationBody` interface continually.

## Solution
~~~
type OperationBody interface {
Validate(*storage.LevelDBBackend) error
IsWellFormed([]byte) error
TargetAddress() string
GetAmount() common.Amount
}

type OperationBodyImpl struct {
}

func (opb OperationBodyImpl) Validate(*storage.LevelDBBackend) error {
// To Do default return
}

func (opb OperationBodyImpl) IsWellFormed([]byte) error {
// To Do default return
}

func (opb OperationBodyImpl) TargetAddress() string {
// To Do default return
}

func (opb OperationBodyImpl) GetAmount() common.Amount {
// To Do default return
}

type OperationBodyVotingResult struct {
OperationBodyImpl
Target string `json:"target"`
Amount common.Amount `json:"amount"`
}

func (opv OperationBodyVotingResult) Validate(*storage.LevelDBBackend) error {
// Implementation
}

func (opv OperationBodyVotingResult) IsWellFormed([]byte) error {
// Implementation
}
~~~
Implement default struct about interface.
The default struct define the methods which should be generated.
Each operation type inherit default struct and define the method only needed for it.