Improve external dependencies

[pgoodman]

I think dependency management needs to be improved. Right now the bootstrap.sh script downloads and builds some packages from source. It would be preferable to use existing package managers to get these libraries.

• Provide a script for unpacking and installing XED to a common library directory, e.g. /usr/local/lib. This should allow the build process to avoid nasty hacks related to linking against the third-party folder.
• Use OS package managers (aptitude, homebrew, etc.) to download and install things like protobufs, glog, etc. This should also be used for pip to install the python bindings of things.
• Include the protoc-produced Python- and C++-generated code for using CFG.proto. Changes to CFG.proto should result in these auto-generated files being updated in the repo.
• Download and globally install the LLVM release, assuming it is not already installed.

These steps will make it easier to have a bunch of simple binaries (e.g. remill-opt, remill-lift, etc.) that can be installed to system directories, without needing to reference stuff in the third_party directory. Ideally, this type of change will enable Remill itself to be packageable.

[pgoodman]

DOing this in the issue_51_third_party_code branch.