Move iCal export to service. Closes #157.

Events should be exportable to iCalendar. This functionality is now extracted into a service.
Minor changes to ics_generator API (class method, naming)
Remove Ruby 2.1.0 from build matrix.

Something's up with this version on TravisCI's end.
Modify ics generator spec
Fix ICS generator specs.
Merge pull request #162 from tmcyf/ics-to-service

[RDY] Move iCal export to service