added Timezone support, made Dow mandatory and Seconds optional to support standard GNU/Linux crontabs
Update docs and tests, and clean up implementation a bit.
Merge pull request #18 from robfig/timezone

Timezone
Create .travis.yml
Small doc fixes
Make Seconds optional, to comply with the Unix cron spec.

Day of week is now required - it was always documented as required, but silently
filled in as * if not provided.

Fixes #14
Return an EntryID when adding a job and allow it to be removed.
SpecSchedule.Next: use time.Truncate instead of time.Date

Add tests to verify nightly jobs work as expected.
This fixes an infinite loop when clock falls back for DST
Move some v2 change to master, so I can remove job.