Prevent excerpts from being generated by default

This is the continuation of #1869 and @troyswanson's work. I just adjusted it to the previous PR's comments, fix tests, and update documentation.

---
On a slightly related note, I tried to add the following test but was surprised to find it failing:
```
Scenario: An excerpt when the excerpt_separator is not present
  Given I have an "index.html" page that contains "{% for post in site.posts %}{{ post.excerpt }}{% endfor %}"
  And I have a configuration file with "excerpt_separator" set to "<!-- excerpt_not_matching -->"
  And I have a _posts directory
  And I have a _layouts directory
  And I have a post layout that contains "{{ content }}"
  And I have the following posts:
    | title     | date       | layout  | content                                         |
    | entry1    | 2013-12-25 | post    | content for entry1. <!-- more --> more content. |
  When I run jekyll
  Then I should see nothing in "_site/index.html"
```
Looks like (on master) if I use an `excerpt_separator` that doesn't exist in the post, the entire post is used as the excerpt. Is this expected behavior?