Fixed some errors and added management command

(1) Fixed Error in _subscription_status.html -- line 9 (issue#52)

Line 9 should be:
{% if request.user.customer.current_subscription.plan and request.user.customer.card_kind %}

Currently, it is as shown below:
{% with request.user.customer.current_subscription as subscription %}
{% if subscription %}
{% if subscription.status == "active" %}

(2) Line 6 in _change_plan_form.html - Need to change the word "Your" to "You" (issue #51)

(3) choices in FormPlan are not sorted in alpha order (issue #48)

I made these changes to forms.py

from django import forms
from payments.settings import PLAN_CHOICES
--> sorted_plan_choices = sorted(PLAN_CHOICES)

class PlanForm(forms.Form):
# pylint: disable=R0924
--> plan = forms.ChoiceField(choices=sorted_plan_choices+ [("", "-------")])

(4) Failed syncdb with django-jsonfield. Installed jsonfield and it worked (issue #45)
     added jsonfield to setup.py instead of django-jsonfield

(5) plan_init does not accommodate Trials (issue #47)
    added trial_period_days to plan_init command