modified help text of debug mode
modified memory printing

The number of bits for one word is different depending on CPU systems.

Note: The printing live memory size must be smaller than actually
assigned memory size by OS.  Because, it only presents "live" memory
in the "major memory" of the process.
Merge branch 'master' of github.com:skcho/zooberry
move memory print function from main.ml to vocab.ml
removed warning about type