RepositoryRelease struct: Add Assets field

The release assets are included in the release object, but they are not being
parsed currently. To get the list of release assets, one must use
ListReleaseAssets, which is one extra unnecessary HTTP request.

This commit adds Assets field to RepositoryRelease struct so that the release
assets are immediately available right after calling ListReleases.

Signed-off-by: Ondřej Kupka <ondra.cap@gmail.com>