An initial overview of threads (#1)

* An initial overview of threads

* try to match spec notation

* fix link and clarify alignment

* require natural alignment, rename i32.wake -> wake

* grow_memory reword, clarify wait alignment, comment about shared w/out maximum size

* initial pass at JS API changes

* fix some links

* more cleanup of markdown

* Clarify that a shared buffer is never detached

* remove HasProperty