Update Mongo Schedule entry to support create task dynamically in application still running

[dainer88]

With this change package will be used in app like flask.
I added change to return object saved in db after.

Example use
celery.py

from celery import Celery

config = {
    "mongodb_scheduler_db": "my_project",
    "mongodb_scheduler_url": "mongodb://localhost:27017",
}

app = Celery('hello', broker='redis://localhost//')
app.conf.update(**config)

when needed add task in app

from celerybeatmongo.models import PeriodicTask
from celerybeatmongo.schedulers import MongoScheduleEntry

task = PeriodicTask(name= 'Importing contacts', task='proj.import_contacts')
task.crontab = PeriodicTask.Crontab(minute="30", hour="7", day_of_week="1",
                           day_of_month="0", month_of_year="*")

entry = MongoScheduleEntry(task=task, app=app)
entry.save()

For testing purposes only, post this file, which will be available until the release of this version.

[dainer88]

@zakird you can review this pull request and release this version.