Add setup for Debian installer creation (fixes #48)

Instructions for creating the installer are given in `platforms/debian/README.txt`. The file `platforms/debian/usr/share/doc/proofpad/README.txt` might need an update at some point.  Note that 64-bit Debian systems need the 64-bit ACL2 version, which means the old Linux tarball doesn't work on those systems.

I've created [32-bit](https://dl.dropboxusercontent.com/u/30534657/InstallProofPad32.deb) and [64-bit](https://dl.dropboxusercontent.com/u/30534657/InstallProofPad64.deb) .deb packages, but have only been able to thoroughly test the 64-bit version. Also, because this setup gives ACL2 write permissions, it will try to fix .cert files on first run, which can take a long time and make the program look frozen.