added compose icon
added getCurrentUser() and postTweet() methods to TwitterClient
made User class Serializable, added EXTRAS_KEY, and use "id_str" from the JSON instead of "id"
made Tweet class serializable, added EXTRAS_KEY, and added fromTextAndUser() static method for creating dummy Tweet objects after composing
get the current user on creation of the TimelineActivity so we can use it when new tweets are posted to display them immediately without issuing another network request
added ComposeActivity, and added menu item and Intent to navigate to it and back from the TImelineActivity