Support Visual Studio 2017

To address #41

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

olegsych / T4Toolbox