Added dict & functions for lesson 3
Word-wrapping fixed
Indentation fixed for code
Lesson 3: (James' sections) Fixing typos and trailing whitespace.
Lesson 3: (James' sections) Spacing out paragraphs, slight rewording

Changing references to "arrays" to "lists" as that is what people are
familiar with in this course.
Lesson 3: (James' section) Further example and more spacing out.

Added examples to show how to use `not in` as well as `in`. And
illustrated the multiple values, same key and same key, multiple values
properties more clearly.
Lesson 3: (James' sections) Adding subtitles for dictionary topics

 * Added examples for the `del` keyword.
 * Made a distinction between assignment for the first time, and
   modifying a key
Merge pull request #8 from asQuirreL/james_dev

James' stuff + Editorial stuff.