Apply thumbnail-picker to audio edit view
Use original img instead of thumbnail in gridview
Sqlite database schemas optimization.

1. Enable global foreign key support.

a)    PRAGMA FOREIGN_KEYS=ON; must be executed right after connection is established.

b)    Set foreign key ON DELETE to a default value, which is initialized during the
  system installation. As a result, if a record is deleted, each dependent record
  will be given a default value. Meanwhile, the default records(image:id=1, category:id=1)
  can never be deleted. So, all application level protection can be removed safely.

c)    Because sqlite employs dynamic types, so a column can hold records of any types,
  application level protection is required to guaranty data integrity. For example, if a
  user input from any form is of integer type, int() has to be applied on it.

d)    So far, only action.admin.category is partially modified to be compliment with the
  above framework optimization. They need to be applied on other actions later.
Merge branch 'master' of http://git.oschina.net/luohao-brian/phonecms