Can we make the trees serializable?

[GoogleCodeExporter]

The current implementation is not serializable. If we load a huge amount of 
data each time when starting, this may limit the usage. However, if it is 
serializable, we can load it once and serialize the entire tree onto the disk. 
During the start-up time, we only have to de-serialize it to load the whole 
tree quickly.

Original issue reported on code.google.com by [email protected] on 25 Mar 2013 at 2:29