Support method or func action.
Use ajax select for User field default style.
Add django comment component.
Add Generic Inline admin support. Fixed #42

Generic Inline Admin Demo:

```python
# Use django Comment Model for example
from django.contrib.comments.models import Comment

class CommentInline(object):
    generic_inline = True  # important to tell xadmin is a generic inline admin
    ct_fk_field = "object_pk"  # and ct_field for content_type field name, default is 'content_type'
    model = Comment
    extra = 1
    style = 'tab'  # other values: `one`, `table`, `accordian` or just forget this attr

class XXXAdmin(object):
    ...
    inlines = [CommentInline]
```