browserify support

This preview release adds some new features, fixes missing pieces of recent features, and most of all, makes it possible to include raster.js in another web project using browserify.

Features:

• Browserify support, raster.js can be included in other projects
• Tilesets can be animated, and saving a gif works
• Interrupts allow for rasterization effects
• Preliminary implementation for attributes
• Zoom affects saved pngs
• ra.then waits for content to load on the web