Support project loading in any version of Visual Studio

This change will allow the RoundTripVSIX project to be loaded in any
version of Visual Studio.  This will be done without Visual Studio
attempting to upgrade the project.

The change really consists of two parts.  The first is the setting of
the MinimumVisualStudioVersion.  This needs to be done once per version
of Visual Studio beyond 2010.  The MinimumVisualStudioVersion property
doesn't affect the build in any meaningful way that I'm aware of.  But
the upgrade wizard does look for this when deciding whether or not an
upgrade is required.

The second change is to choose the SDK targets file based on the current
version of Visual Studio.  This is done by keying off the
$(VisualStudioVersion) property to build up the $(VsSdkTargets)
property.  The name of this property isn't important (other than
choosing one that Visual Studio isn't using).  The import of the SDK is
then switched to use this variable and be conditional on the SDK
existing.  The Conditional attribute is a must have here.  If it is
missing the upgrade wizard will flag this project for needing an upgrade

There are still several problems with the project though.  In particular
it still identifies the Visual Studio assemblies it references by a
non-strong name.  This means a build in 2012 will bind to the 2012
version of the assemblies.  These are built for the 4.5 framework and
hence can't be used in a 4.0 project.  This will result in a lot of
build errors.  It also has other problems building if the 2010 SDK isn't
installed.  The next commit will begin to take care of these problems
Switch to strongly named references

The RoundTripVSIX project can now be edited in a variety of Visual
Studio environments.  Each of these environments want to provide
different versions of the SDK references we are using.

It's important to guarantee that we are always using the reference
assemblies shipped with Visual Studio 2010.  These are fully supported
when loaded in newer versions (via redirects in devenv.exe.config).
Also all of the newer references are bound to newer versions of the .Net
Framework.  The RoundTripVSIX project will always target 4.0 hence even
if the new ones are bound to we can't reference them.

With this change we can now build the extension in any version of Visual
Studio so long as the 2010 SDK is installed on the machine.  That's
still pretty limiting though because the SDK requires a full Visual
Studio install to work.  The next commit will fix by deploying reference
assemblies with the source
Allow for build without full SDK installation

In this change we deploy the reference assemblies which are needed to
build the RoundTripVSIX project.   These are the reference assemblies
from a 2010 installation. All versions of Visual Studio beyond 2010 have
appropriate binding redirects for these values in the devenv.exe.config
file.  Hence our extension will load just fine if it references these.
This is generally true for any assembly that Visual Studio includes in
the SDK

The only change necessary to the project is to include this directory in
the reference search path.
Enable F5 debug everywhere

The final change we need is a small tweak to the .csproj so that F5
works out of the box.  This is done by adding the appropriate start
action to the .csproj to run devenv under a different registry hive.

Visual Studio generates this information in new VSIX projects but it
does so in the .csproj.user file.  Virtually every source code control
system excludes this file by default hence it will work until you clone
the source onto a new machine.  By putting this directly in the .csproj
file we ensure F5 will work for anyone who clones our source code