Scrolling platform generator

# Changes #

### index.html ###
- added template for platforms

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### main.js ###
- interval loop to create a platform view

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### platform-model.js ###
- created a model and collection constructor for the platforms
- model will randomly generate a y-coordinate and width for each platform
- ***need to figure out what else we need to add to the model for managing the information we'll need for collision detection***

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### platform-view.js ###
- created a view constructor for the platforms
- view destroys itself after a set timeout interval

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### sprite-animation.js ###
- just cleaned up the file a bit

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### _variables.scss ###
- added a file for Sass variables
- currently only contains some colors

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### main.scss ###
- added platform class
- platform animates with keyframe from right to left