Ensure add-apt-repository exists

With trusty, when installed via PXE, it's possible to end up
without add-apt-repository in the target.  This ensures it is
available.  Unfortunately, it is provided by different packages
across different distributions.
Clean up kernel arguments passed to stage2

Remove support for installing distros older than Precise via PXE,
and standardize the kernel arguments.  For anything we don't know
about, ask the user rather than guessing wrong.  This still allows
the installation of ancient distros.
Replace svnversion with the short hash

We're no longer using Subversion; the stage1 installer should
be versioned using the git hash.  We use HEAD since if you're
running "make install", it will operate on HEAD.

Also, there is no longer a need to exclude .svn when
creating the tarball
Support PXE installs of Trusty

Initial checkin of PXE support for Trusty.  Configuration
is mostly identical to Raring and Precise, except that we
no longer need to exclude .svn from tarballs.