Asset retrieval is independent of OS Path

We noticed that when creating the asset table you are forcing the keys to be Linux-style paths. However, when we tried running our binary with generated asset files under Windows, we would get errors because we are using the `filepath.Join(...)` to generate the key for the `Asset(...)` command. This is because in your `var _binddata` map your are keeping the keys in Linux style paths (with `/`).

For instance we create the asset called "foo/bar.go" on Linux, when we call it on Windows using the `filepath.Join(...)` to generate the key, we get "foo\\bar.go" and the `Asset(...)` method fails. This pull request enables us to use keys generated with `filepath.Join(...)` regardless of OS.

Best,

@XenoPhex and @maximilien