Use the upstream V8 repo rather than v8-native-prototype.

[ncbray]

Fixes #12

[ncbray]

@flagxor - thinking about this, should it be a git submodule or a pinned hash, ala LLVM? If it's a pinned hash, it would be easier to modulate between downloading binaries or building from source. Are we still planning to pull d8 binaries on the bots?

[jfbastien]

As discussed offline with @ncbray I agree with him and think it's easier to have git hash: I'd rather use the submodule file format and switch from submodule sources to downloaded at will (the tool that gets binaries would just parse that file), but git submodule won't be happy with only a few of the submodules being initialized and the other being downloaded binaries. If we want to switch only some of the dependencies to binary during our workflow then submodules aren't a good fit.