Pub, and breaking changes

[Dominic Hamon]

Hello

As part of the move to introduce pub support, there have been a fair few changes to the layout of the project. These mostly won't affect anything, but one will:

lib/box2d.dart is now the platform-agnostic version

lib/box2d_browser.dart is now the version that runs in the browser

This is true as of v0.0.3 (via pub or git tag).

- dominic