v6.0.0-alpha.13 release of PowerShell Core

With 99 non-merge commits since v6.0.0-alpha.12, here are the major changes:

1. Fix NullReferenceException in binder after turning on constrained language mode
2. Enable Invoke-WebRequest and Invoke-RestMethod to not validate the HTTPS certificate of the server if required.
3. Enable binder debug logging in PowerShell Core
4. Add parameters '-Top' and '-Bottom' to Sort-Object or Top/Bottom N sort
5. Enable Update-Help and Save-Help on Unix platforms
6. Update the formatter for 'System.Diagnostics.Process' to not show the 'Handles' column
7. Improve Write-Progress performance by adding timer to update a progress pane every 100 ms
8. Enable correct table width calculations with ANSI escape sequences on Unix
9. Fix background jobs for Unix and Windows
10. Add Get-Uptime to Microsoft.PowerShell.Utility
11. Make Out-Null as fast as '> $null'
12. Add Dockerfile for windowsservercore and NanoServer
13. Fix WebRequest failure to handle missing ContentType in response header
14. Make Write-Host fast by delay initializing some properties in InformationRecord
15. Ensure PowerShell Core adds an initial '/' rooted drive on Unix platforms
16. Enable streaming behavior for native command execution in pipeline, so that 'ping | grep' doesn't block
17. Make Write-Information accept objects from pipeline
18. Fixes deprecated syscall issue on macOS 10.12
19. Fix code errors found by the static analysis using PVS-Studio
20. Add support to W3C Extended Log File Format in Import-Csv
21. Guard against ReflectionTypeLoadException in type name auto-completion
22. Update build scripts to support win7-x86 runtime
23. Move PackageManagement code/test to oneget.org
