support sub-directory in the image folder

[ppodxiang]

I have the main branch using a simple photo folder: it have 13 .jpeg files in ./ and 13 .jpeg in ./2015_12 folder:
.
├── 2015_12
│   ├── 20151201_IMG_4403.JPG
│...
│   └── 20151231_IMG_4435.JPG
├── DSC01938.JPG
...
├── DSC01943.JPG;
the start_web.py return 404 for all files in ./2015_12 folder as it direct join image directory and filename, omit the relative path.

get_file_path_from_db(filename) use input filename and compare with info in the database and return absolute full file path.

WARNING: This may slow down the process when database is rather large!