upd: ical-tools to work with rSchedule

This branch updates the ical-tools package to work with the new rSchedule api.

• Updates @rschedule/ical-tools to depend on the ical.js package for parsing iCal strings to JSON (actually jCal)
• Adds VEvent object which aligns with the VEVENT component in the ical spec.
  • VEvent objects, as spelled out in the iCal spec, only support one RRule and EXRule property.
  • The dtstart date is specified for the VEvent, rather than individual rules.
  • The dtstart date is always the first occurrence (unless an exdate / exrule removes it).
  • The timezone of the VEvent is always the same as the dtstart property.
• Adds enhanced iCal string parsing / serializing functionality that more closely aligns with the spec.
  • Can now parse entire VEVENT components inside iCal strings.
  • Still cannot parse VCALENDAR components.
  • Still does not support all aspects of the VEVENT spec (e.g., the VTIMEZONE component is not supported).

Addresses #11 (closed), #10 (closed), #9 (closed)