add PingApi

that gives us JavaScript methods to interact with the Ping API.
The following methods do exist so far

```js
// initialize the API
api = new PingApi()

// you can optionally pass a custom endpoint for the api
api = new PingApi('http://localhost:8880/api')

// sing in / out
api.account.signIn('joe@example.com', 'secret')
api.account.signOut()

// contacts
api.contacts.findAll()
api.contacts.add({ name: "joe", phone: "+254 716 123 456"})
api.contacts.find(123) // pass ID of existing contact
api.contacts.update({name: 'funky joe'})
api.contacts.remove(123)

// broadcasts
api.broadcasts.findAll()
api.broadcasts.add({ contact_ids: [123, 456], message: 'you okay? '})
api.broadcasts.find(123)
```

Every method returns a [jQuery promise] as it's returned from jQuery's
[ajax method](http://api.jquery.com/jQuery.ajax/). For example

```js
api.account.signIn('joe@example.com', 'secret')
.then( showWelcomeAlert )
.fail( showError )
```