v2.5.7 - Bug fixes

• Store total app sessions, total app session length and first app open date as user properties, parity with ObjC
  #283

• Add NSString as MixpanelType thanks @eXhausted
  #265

• Fix crashes
  #282 thanks @RamblinWreck77
  #273 thanks @drfindley
  #188 thanks @kevinchau

• Optimize .reduce usage on Array, thanks @asiliuk
  #281