Add hashtable

This is a slightly modified version of Dustin's 'genhash'. The
modifications were primarily to fix up some build issues.

There's also a wrapper function to create common hash tables (i.e. one
which doesn't copy keys/values).

Change-Id: I7fdb9abac62fe92acc684e7c5afb6f445436b3b0
Reviewed-on: http://review.couchbase.org/27563
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>