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
Merge pull request #1 from recadra/v2

V2