Converted Module::Build to use Meta 2.0 inside

This converts Module::Build to use Meta 2.0 inside, closing #18. This fixes a number of issues such as #14. This has a number of backwards compatibility issues, in particular:
 * `get_meta` now returns meta 2.0, so anything assuming 1.x may break
 * `meta_merge` and `meta_add` are limited to items that can easily be upgraded to meta 2.0, unless they're declared to be 2.0. This includes prereqs (which is a stupid thing to meta add/merge anyway).