Addition of some popular features

1) Seconds field is now optional and Dow field is mandatory to support standard GNU/Linux crontab format.

2) An optional TZ=<Location> clause (ex : TZ=Europe/London) in the beginning of cron spec to support specification of cron times in that location rather than in the local timezone

Sample usage :
c := cron.New()
c.AddFunc("TZ=Europe/London 07 21 * * *", func() { fmt.Println("Runs at 21:07 London time") }, "London test") 
c.Start()

3) Added a function NextNTimes for Entry struct which returns next (upto) N activation times.

4) Added DST support and test cases from github.com/lukescott/cron feature-dst branch

5) Added safe Remove() and its test cases.