Main functions implementation

Two new classes named Item and User are added to represent users and
items. Changed some werid interface function names to improve
readability.
The synchronization priciple is, whenever a write or set operation
occurs, we keep the execution flow atomically. There'll be no lock for
the reading so far.
TODO: Adding the functions that connects the user and users' items and
the items in "cart" (here pool may be a better name).

In addition the former mysql databased structure was abandoned, due to
the stupid and time consuming driver installation.

Signed-off-by: Chen, Yiqun