Add support for round screens, color and SDK 3

[llewelld]

Make changes to support round screens (using text_flow), color and upgrade the build script to SDK 3.

The changes have #ifdefs to support rectangular screens and B&W displays, but it was necessary to make some changes to the original layout, so it the result doesn't look exactly the same on the original pebble. This will also add a progress bar for when headlines/stories are downloading to the pebble.