Design Assumptions:
1. the number of directories are magnitudely smaller than the number of files
2. unlimited number of files under any directories
Phylosophy:
  metadata for directories and files should be separated
Design:
  Store directories in normal map
    all of directories hopefully all be in memory
    efficient to move/rename/list_directories
  Log directory changes to append only log file
  Store files in sorted string table in <dir_id/filename> format
    efficient to list_files, just simple iterator
    efficient to locate files, binary search

Testing:
1. starting server, "weed server -filer=true"
2. posting files to different folders
curl -F "filename=@design.txt"   "http://localhost:8888/sources/"
curl -F "filename=@design.txt"   "http://localhost:8888/design/"
curl -F "filename=@directory.go" "http://localhost:8888/sources/weed/go/"
curl -F "filename=@directory.go" "http://localhost:8888/sources/testing/go/"
curl -F "filename=@filer.go"     "http://localhost:8888/sources/weed/go/"
curl -F "filename=@filer_in_leveldb.go"     "http://localhost:8888/sources/weed/go/"
curl "http://localhost:8888/?pretty=y"
curl "http://localhost:8888/sources/weed/go/?pretty=y"
curl "http://localhost:8888/sources/weed/go/?pretty=y"