Add support for :period parameter in Videos.top

[schmich]

• Add specs
• Update docs
• Videos.top(:period => :week)
• Valid: :week, :month, :all
• GET /videos/top
• https://github.com/justintv/Twitch-API/blob/master/v2_resources/videos.md#get-videostop

[schmich]

This is covered by issue #39.