Fixed typo

Fix mac build error

is => it

Also added simple (in-process) test.

Change-Id: I38580d554dd52f590e3396ec4846e07546dcf07d
Tested: on Linux.

Added new constants

Add new constants in Constants.java

All single ints int FlatBufferBuilder.java were replaced by constants
from Constants.java

pulling methods inside the class... so as to share wrapInNamespace later