Allow massadmin to be used with a custom AdminSite

[ecederstrand]

Django allows customization of AdminSites, as described in https://docs.djangoproject.com/en/1.9/ref/contrib/admin/#customizing-adminsite
This patch allows massadmin to work with a custom AdminSite by passing the custom site to the view (it is also necessary to add the mass_change_selected action to the custom site):

from massadmin import mass_change_selected

admin_site = MyCustomAdminSite(name='custom_admin')
admin_site.add_action(mass_change_selected)

url(r'^admin/', include(massadmin.urls), kwargs={'admin_site': admin_site}),

[coveralls]

Coverage remained the same at 91.111% when pulling 6e66887 on ecederstrand:patch-1 into 38b2b28 on burke-software:master.

[PetrDlouhy]

@ecederstrand Could you please also add test for this functionality?

[ecederstrand]

Sorry for the late reply. Of course a test should go with the patch, but 'm unfamiliar with your test suite and unsure how to go about testing this. Could you nudge me in the rigt, general direction?

[PetrDlouhy]

@ecederstrand I finished this and merged in #76, so I am closing this.