Ignore .zip files and the hterm directory.
Automate fetching and incorporating hterm into the app.

This adds Chromium's "assets" repo as a submodule. build.sh ensures that is
initialized and updated, makes the "dist", and copies the relevant files into
the app directory.
Change from vt100 to xterm-256color to match hterm.
Track naclports changes.
Major refactor cleanup. No new features, just new bugs.

Cleans up how file descriptors are managed. Also removes a lot of "unistd"
logic out of pepper_wrapper.cc and into a generic implementation.