[resource_pack] Attempt to fetch texture pack using XHR, at least if local

[deathcap]

resource_pack.js logs server-offered texture packs, since it cannot know whether or not they are downloadable, but optionally as an enhancement it could try to download and automatically install.

This would fail if the URL was remote and server has not enabled CORS, but if https://github.com/deathcap/wsmc/issues/44 voxel-clientmc serves up textures, then it the URL would be in the same domain as the voxel-clientmc web client, so the file should be downloadable without crossing origins.