Add build.OperationBuilder interface

[bartekn]

Consider the following code:

var operationBuilder interface{}
if !destinationExists {
    operationBuilder = b.CreateAccount(mutators...)
} else {
    operationBuilder = b.Payment(mutators...)
}

// Check errors here

txBuilder := b.Transaction(
    operationBuilder.(b.TransactionMutator)),
)

If we wanted to check if there were errors after applying mutators (Err field in builders) it's not possible because type assertion works for interfaces only. To solve this we can create an interface:

type OperationBuilder interface {
    MutateTransaction(o *TransactionBuilder) error
    Mutate(muts ...interface{})
    Error() error
}