v6.0.0-alpha.14 release of PowerShellCore

Here are the major changes:

- Moved to .NET Core 1.1
- Add Windows performance counter cmdlets to PowerShell Core
- Fix try/catch to choose the more specific exception handler
- Fix issue reloading modules that define PowerShell classes
- `Add ValidateNotNullOrEmpty` to approximately 15 parameters
- `New-TemporaryFile` and `New-Guid` rewritten in C#
- Enable client side PSRP on non-Windows platforms
- `Split-Path` now works with UNC roots
- Implicitly convert value assigned to XML property to string
- Updates to `Invoke-Command` parameters when using SSH remoting transport
- Fix `Invoke-WebRequest` with non-text responses on non-Windows platforms
- `Write-Progress` performance improvement from `alpha13` reverted because it introduced crash with a race condition
