Add project-watch to discover projects with file-notify

Projects can be created outside of Emacs, but users might want to be
able to switch to them with project-switch-project immediately.  This
function supports that.  If a user calls (project-watch "~/src" 1) then
any projects under ~/src will be discovered automatically immediately
after their creation.

I have strived to make this function usable for a wide set of use-cases,
I think it will be a useful addition to project.el.