[Done]
1. (tempalately)Use user.json to specified user identity.
2. Server open the material folder depends on user.For example, AOOP's permanent ID is UEE1303，and I took this class in 102B(current ID:1060)，so when I
   view the AOOP material，what I see would be UEE1303/1060.
3. Fixed:Failed to get JSON file property .(\handlers\materials.js :Line 92)
4. Be able to get file names in dir,but failed to load it to the page. View [Wait for improved-2].
[Wait for improved]
1. function load_material should be rewrite with async.waterfall or something else,rather than nested structure.(\handlers\materials.js :Line80-148)
2. Failed to read files from dir.

[Wait for discussion]
1. Should we implement it like [Done.1]?If so,continue to 3.
2. Should we implement it like [Done.2]?
3. Improve the format of user json.