Add API request for StackOverflow questions.

* Add QuestionList and Question entities to represent the API response.
* Add Server interface to define the StackOverflow API endpoints to be used.
* Add QuestionCallback to MainActivity.
* Update MainActivity's onCreate to fire off a StackOverflow /questions api request.
Display StackOverflow /questions api results in ListView.

* Create res/layout/list_item_question.xml layout for displaying the Question row.
* Create QuestionListAdapter to display a Question object in the list_item_question.xml row layout.
* Update MainActivity to wire up a QuestionListAdapter, the ListView, and the data returned from the /questions API call.
Rx-ify /questions API call.

* Add rxandroid dependency in build.gradle.
* Update the original Callback-based Retrofit API call with Rx Observable-based approach.
Merge pull request #2 from vwmattr/rxandroid

Add RxJava + Retrofit API call example