Fix auto-regeneration and ignore Listen events during compilation.

I believe it would lead to a lot of problems if Listen built the site
while the site was already being built in another thread.
Use a better method name for displaying the wait page.
Add :no_render option to turn off initial site generation.

The :no_render option is useful for cases where the site has been
previously built by another operation.  For example, if the site has
been built by a continuous integration environment and synced over.