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major change to goassets

Using the idea of a custom filesystem for testing (see the `10 things you (probably) don't know about go` talk) the
assets code was rewritten:

* If the GOASSETS_<pkg_name>_PATH is set that is used for retrieving the files.
* Otherwise there is a filesystem like interface for the assets. Use something like `Open("asset_name")` and the
  `Read` method of the returned object.