README: expand section on building debian packages

Removed "make install" instructions because its behavior may be surprising
compared to standard autoconf projects.
add libpcap-dev dependency to setup.sh and README
add INTERNALS.md

This document is for details useful to new developers but too in depth for the
README.
Merge into master from pull request #8:
documentation improvements (https://github.com/floodlight/ivs/pull/8)