Improve rendering strategy of activities
Add get_for_model/get_for_object
Improve test cases

snapshot is no longer defered.
Add get_activities_of templatetag

{% get_activities_of <object> as <variable %} will collect activities
related to the specified <object> into <variable>

{% get_activities_of '<app_label>.<model_name>' as <variable> %} will
collect activities related to the specified model (by <app_label> and
<model_name>) into <variable>
Add activities test execution on Tox
Fix a message of TemplateSyntaxError
Add 'get_template_extension(typename)' method

Add

- `activities.mediator.ActivityMediator.default_template_extension`
- `activities.mediator.ActivityMediator.template_extensions`
- `activities.mediator.ActivityMediator.get_template_extension(typename)`
- `django.conf.settings.ACTIVITIES_DEFAULT_TEMPLATE_EXTENSION`
- `django.conf.settings.ACTIVITIES_TEMPLATE_EXTENSIONS`

to control template extension by `typename`.
Merge pull request #289 from kawazrepos/improve_activities

Improve activities