Remove platform-specific rint implementation.

[non]

By upgrading scala.js to 0.6.5, we can just use java.lang.Math.rint and let scalajs wire it up to a correct implementation.

This also allows the std module to become a "pure" cross project, removing the need for js/src, jvm/src, and shared/src.

Fixes #74. Review by @InTheNow.

[johnynek]

Looks good to me, but I don't really know anything about how scala-js compilation works, so it is tough to see the impact of these changes.

[ghost]

LGTM :+1:

[non]

Thanks!