Hopefully, the Chisel core is now thread-safe.

Edit/Add scaladoc and site targets to Makefile.
Add site, ghpages objects to build.sbt and project/plugins.sbt

The implementation is a total kludge, but at least it's not broken.

It now fails if the width is unknown or is not equal to 1.
We could consider relaxing this later, defining it as this.orR.

Doing so results in incorrect code for Bools, because the widths of some
Bool primops' results are greater than 1.

The alternative would be to make Bool not extend UInt.

I had inadvertently disallowed it.

The previous implementations were incorrect for large N.

This probably isn't a good idea, but Emitter.initPorts depends on it
to create the literal 0.

This simplifies the handling of empty when statements.