format links to external pages. some misc md style improvements

**This commit is to simply replace things like:**

Read this long article on code: http://super.long.com/article-written-about-code

**with:**

Read this [super long article on code](http://super.long.com/article-written-about-code)

**Overall, course content was not changed, but in some instances a sentence was re-worded to make the flow of links to outside resources more natural.  Some instances were changed to a list format.**

These changes to not conflict with the pending PR #65 (Add chapter 9), so it's ready to merge!