Excerpts

So far, using the excerpts feature in the blog was not straightforward, and required  the user to customize the templates. See [here](http://www.blogofile.com/demo/post_excerpts.html) for explanation.

This patch handles the following two issues:
1. The chronological template will automatically use the correct form (excerpt/content) depending on the value of "blog.post_excerpts.enabled" in _config.py.
2. Based on pull #26 of nryoung, blogofile will fall through to bf4 if lxml is not installed. A discussion [here](https://groups.google.com/forum/?fromgroups#!topic/blogofile-discuss/7Y7eXWOH29o) have more information on the reason for this.

I am aware that you are travelling, so do not worry about reviewing this pull request fast.