给材料中心增加标签支持 #1

* 添加 django-taggit 依赖
* 增加 MaterialTag 和 TaggedModel 模型
* 相应的 DB 迁移
Merge branch 'material-tag-support'
给材料中心增加标签支持 #2

* 在模板中渲染材料标签
Merge branch 'material-tag-support'
诊断奇怪的错误

TemplateSyntaxError at /app/materials/list/

Caught FieldError while rendering: Cannot resolve keyword 'None' into
field. Choices are: creator, id, is_official, materialentry, name, slug,
taggedmaterial

这很奇怪啊。。为什么是模板渲染错误呢？
Merge branch 'material-tag-support'
修正忘记添加 related_name 造成的奇怪错误

根据 Django 官方文档：
https://docs.djangoproject.com/en/1.3/ref/models/fields/#django.db.models.ForeignKey.related_name

    Note that you must set this value when defining relations on abstract
    models.
Merge branch 'material-tag-support'
Merge branch 'master' into production