Add forEach(), groupBy(), hashCode(), includes(), notEquals(), unique(), uniqueBy()



• Add forEach()
• Add groupBy()
  • object and arrays cannot be grouped on deeply equal keys because plain objects cannot have objects as keys
• Add hashCode()
  • non-Immutable.js values are JSON stringified
• Add includes()
• Add notEquals()
• Add unique() (#51)
• Add uniqueBy() (#51)
• No breaking changes