Transport selection

Replace ContextMenu with popup to temporarily change preferred transport on longpress of send button.

![screenshot](https://cloud.githubusercontent.com/assets/373823/3654788/8b2549d0-1170-11e4-8ea2-70ec26f0abe9.png)

Android's PopupWindow doesn't even extend View, making this not really a view as much as it as a "popup manager" out of necessity.

Because of this, the proper segmentation might be to create a CompositionView that manages all the UI for the compose text, send button, popups, etc. What do you think about that?