Couple modifications to "next date" retrieval.

In requesting the next date, we try to figure out which date that is.
There were a couple issues with the current implementation.

When you
start with a real date, using that as a "pattern" to find the next date
from is next to useless...no no, it is useless. So, instead of finding
the next date we just check the value and warn the user if its in the
past, otherwise we return it as a new date object (wrapped in a time
Date in case the user has setup timezone info).

When the source is actually a cron pattern, we want to make sure that we
are getting the next appropriate date to actually trigger on. The
original implementation of this code used while(1). This was changed in
8c158dc886 but I'm not sure what you really get out of it other than it
artificially limits a potential inf. loop (what I don't get is the
potential for it...seems very unlikely).

Added some test cases to check these cases.

Signed-off-by: Nick Campbell <nicholas.j.campbell@gmail.com>
Fix documentation.

GH-59
adding mongodb backuping support ep2; issue #60
Merge branch 'master' of github.com:chester1000/node-cron