Third tutorial revision

[uranusjr]

Things to revise after Django Girls Taipei v2: (most of these can be found in the slides)

• Mention ValueError for int('Hello!') after introduction to type conversion (before going into variables).
• Wording at the end of the variable section (mentioning NameError) needs modification.
• Mention IndexError in list section.
• Add introduction to import other files.
• Add a very simple introduction to regular expression.