MessageBuffer.slice()

Current v07-draft branch uses limit() setLimit(...) methods to obtain the written data range. Instead, we should provide MessageBuffer slice(int offset, int len) method to make MessageBuffer immutable (except its memory contents).

Fixes #90: Using MessageBuffer.slice() instead of using setLimit

I simply removed limit parameter from MessageBuffer, and provide slice() method. Memory management of generated slices need to be discussed in another ticket.