Lettopia eBay API

Reason

- The original code has ebay api coupled into the code, which makes it hard to do unit test.
- Make it a seperate package also help us to reuse it.
- Decouple the code will help ebay.js looks more clear.

Class

$3

A singleton in the application. It takes appId, devId and certId secret to construct. Provide the following functions:

#### [x] GetOAuthToken: For OAuth

#### [x] GetRefreshToken: For OAuth renew

#### [x] GetSessionId: For Auth'nAuth

$3

It is like eBay seller. It takes token to construct. There are two authentication methods provided by
eBay, i.e OAuth and Auth'nAuth which you have to specify when you create it.

#### [x] GetOrders: For getting ebay order

#### [x] GetSellerList: For getting ebay seller products

#### [x] GetUser: Get account when create ebay user

#### [x] CompleleSale: Update tracking number

#### [x] SetNotificationPreferences: Set the user notification

#### [x] AddFixedPriceItem: List item to ebay

#### [x] GetUserPreferences: Get user preference including its listing limit

#### [x] GetMyeBaySelling: Get seller's selling

#### [x] SetUserPreferences: Set the user's preference, including Out-of-Stock feature

#### [x] ReviseInventoryStatus: Used for inventory sync

#### [x] ReviseFixedPriceItem: Set the item to GTC listing for out-of-stock feature

$3

Parse info from the recieved ebay notification

General flow of calling eBay API:

1. Build XML request by API name and authentication type. xml-builder
2. Send the request to ebay endpoint.
3. Parse the response into json. xpath is a good way

Test

Use mocha as the unit test.