Serve 404 error on the initial blog state fixed

This commit fixes the bug, which led to 404 error when the serve command was
invoked before building a blog. The check for an output directory was added,
so now serve rebuilds blog if it hasn't been done before.

Fixed #33
Merge pull request #39 from ikalnitsky/serve-prebuild-fix

Serve 404 error on the initial blog state fixed