Addin: Change project file to open in every VS version

Still requires the appropriate SDK to be installed.
Thanks @sharwell for much simpler implementation
Addin: Replace VS references with @sharwell's v10 NuGet packages

These are installed using Automatic Package Restore

As long as we reference all VS assemblies from this
package and make sure to stick to v10 packages, the
addin should work in every version of Visual Studio
starting with 2010.
Addin: Port VSIX manifest to v1 to enable installation on VS2010
Addin: Fix runtime error in VS2010

explorer.SelectedItems returns object[]
Addin: Include all dependent assemblies in VSIX

Otherwise, ILSpy will fail to start if it isn't already running.
This bloats the VSIX to nearly 6MB, but there is no alternative.
Addin: Retarget to .Net 4.0

The addin will now work on VS2010 without installing .Net 4.5.
Merge pull request #513 from SLaks/addin-version-compat

Addin: Add support for VS2010+