Added parsers for movies opening this week, and this week's box office top 10 (closes #2)

Introduced 3 new classes to parse http://www.imdb.com/movies-in-theaters/ page:
- InTheaters < List
- OpeningThisWeek < InTheaters
- BoxOfficeTop < InTheaters

Also, refactored List and Top classes a bit, and added a method to strip year (in string_extensions)