Watch improvements

For dealing with template files, we now only re-generate the one that
changed instead of all of them.
Remove livereload

This doesn't work reliably enough given the current build setup.
Adjust clean task

Changes this to clean the build/ directory contents rather than the
build directory itself -- this keeps the GAE SDK server happy by not
forcing it to lose its reference to the directory if it's already
running.
Mustache variable clean-up

These now use a more consistent naming across different Mustache types.
Adjust 'partials' watch task

Because partials also affect the error and redirect markup, the tasks
should reflect that.
Fix rebuild on sitemap change

This did not have reload: true wrapped properly, so sitemap changes did
not affect the build.