Allow addAsset usage on native targets
Include contents of private/ as server static resources
Factor out self.static into Target
Clean up interface to addAssetDir
Specify static resources directory in server manifest

Also a few bug fixes (private/ directory was being scanned for source files,
private/ files were being included in any server target including ctl).
Add staticDir field to load object in server's program.json
Include private/ files in target's static/ dir
Add test for static server assets packaged correctly
Make public asset test slightly less hardcode-y
Remove unnecessary "static" field on program.json

We don't need this now that each load item has its own staticDir.
Add functions for reading text or binary assets.

Assets are namespaced by package, so packages can only read their own files.
Sort of WIP... To do:
1.) Don't allow ../ in file paths?
2.) Async versions of the API
3.) Make this API available on the client?
4.) More tests
Add async version of Assets API
Just use Uint8Array always for getAsset data
Clean up (mostly comments)