Fix nil pointer dereference when accessing AccountEvent event header account

[weppos]

Closes #83

This is a tricky issue. The bug is caused by a naming conflicts in the JSON deserialization of an account even payload.

An AccountEvent is defined as:

type AccountEvent struct {
	EventHeader		AbstractEvent
	Data    *AccountEvent     `json:"data"`
	Account *dnsimple.Account `json:"account"`
}

This is an example of struct composition in Go. The EventHeader is defined as:

type EventHeader struct {
	APIVersion string   `json:"api_version"`
	RequestID  string   `json:"request_identifier"`
	Actor      *Actor   `json:"actor"`
	Account    *Account `json:"account"`
	Name       string   `json:"name"`
	Auto       bool     `json:"auto"`
	payload    []byte
}

The combined AccountEvent struct is:

type AccountEvent struct {
	APIVersion string   `json:"api_version"`
	RequestID  string   `json:"request_identifier"`
	Actor      *Actor   `json:"actor"`
	Account    *Account `json:"account"`
	Name       string   `json:"name"`
	Auto       bool     `json:"auto"`
	payload    []byte
	Data    *AccountEvent     `json:"data"`
	Account *dnsimple.Account `json:"account"`
}

When go tries to parse the payload, the two json:"account" definitions conflicts each other. The issue arises from the fact the AccountEven is a flatten representation of an Even payload that is actually a nested structure.

In Go parsing nested structures is tricky. We took the path to represents events as individual types.

This PR prefixes the event header fields with Event (so Actor becomes EventActor). The event header is parsed independently, and later assigned.

[weppos]

Replaced by #85