VS2014 support in VSExtension

Ability to install/uninstall vsix packages through VsixPackage element from the VSExtension.

Very straight forward following of the old code as the vsix installation did not change from VS2013 to VS2014.
The intent  of not having a year in the variable was to include a registry search for each supported version. You should only need to find the latest (or potentially any) VSIXInstaller so you only need one property anyway.