Handle async loading of google maps API

[mebibou]

It is actually not necessary to directly initialize in the linking function the google maps API services, therefore we could only do it when we actually use them.

This fixes #69

[ghost]

This returns a console error:

angular.js?v=0123456789&m=6:11706 Error: [$injector:undef] Provider 'googlePlacesApi' must return a value from $get factory method.
http://errors.angularjs.org/1.3.20/$injector/undef?p0=googlePlacesApi
    at angular.js?v=0123456789&m=6:63
    at Object.enforcedReturnValue [as $get] (angular.js?v=0123456789&m=6:4074)
    at Object.invoke (angular.js?v=0123456789&m=6:4219)
    at angular.js?v=0123456789&m=6:4037
    at getService (angular.js?v=0123456789&m=6:4178)
    at Object.invoke (angular.js?v=0123456789&m=6:4210)
    at angular.js?v=0123456789&m=6:6547
    at forEach (angular.js?v=0123456789&m=6:326)
    at Object.<anonymous> (angular.js?v=0123456789&m=6:6545)
    at Object.invoke (angular.js?v=0123456789&m=6:4219)

[mebibou]

It would mean that you are calling the directive before having loaded the google places api from google. I guess I am not, which is why I didn't see this error.

This doesn't happen when you still load synchronously right? only when async?