Support Visual Studio 2017
- Increment extension version from `14` to `15` in assembly versions and `.vsixmanifest` to because new package can be installed only in Visual Studio 2017
- Upgrade all projects files
- `ToolsVersion="15.0"` to prevent Visual Studio 2017 attempts to upgrade VSSDK projects.
- Remove hard-coded `Version=14.0.0.0` from Visual Studio assembly references to automatically resolve them to `Version=15.0.0.0` of Visual Studio 2017.
- Reference Visual Studio 2017 implementation assemblies with `15.0` suffix instead of `14.0` of Visual Studio 2015.
- `<TargetFrameworkVersion>v4.7</TargetFrameworkVersion>` to stay on the latest framework version.
- Use relative paths instead of `$(SolutionDir)` to support command-line build
- Use latest stable versions of NuGet packages.
- Copy referenced assemblies to build output in test projects because Visual Studio 2017 assemblies are no longer in GAC.
- Fix or ignore broken tests.
- Upgrade `LocalTestRun.testrunconfig` to run integration tests in VS 2017 experimental hive.
- Upgrade `appveyor.yml` to run CI builds on `image: Visual Studio 2017`.
- Cleanup PowerShell tests
- Change `build/Module.tests.ps1` instructions to install Pester from PowerShellGallery.org instead of NuGet.org.
- Remove obsolete `.nuget/` files
- Refresh `README.md`
