WebSandboxMC

• Add a WebAssembly version of the web client, faster and smaller on supported browsers, served up by default with an asm.js fallback for older browsers (GH-64)
• Add web player authentication, /websandbox auth gives the player a unique web link to login under their username in the web client instead of anonymously (GH-48)
• Add /websandbox clear command to remove leftover entities from web players if the server crashed (GH-64)

Update to NetCraft build 447:

• Add specifying username and authentication token on command-line arguments or document hash, an alternate means of authentication used by WebSandboxMC (satoshinm/NetCraft#143)
• Improve performance by serving a separate .data file for resources instead of inefficiently embedding in .js (satoshinm/NetCraft#165)
• Fix missing optimizations in release builds (satoshinm/NetCraft#167)
• Fix division by zero in player count, a latent bug exposed by the WebAssembly port (satoshinm/NetCraft#168)