Create and manage a persistent local file-system for NaCl platform.

Start getting and unpacking the server assets to the local file system
in javascript before starting the NaCl module. NaCl module then only
needs to deal with the local file system. Currently the size of the file
system is fixed at 128MB.
Implement more File functions.

Use a callback function when opening the local file system to make sure
that the file system has been opened before calling NaClInitGame.
Implement File::flush, File::close, File::getPosition and
File::setPosition.