v6.0.0-alpha.8 release of PowerShell

With 304 non-merge commits since v6.0.0-alpha.7,
here are the major changes:

- PowerSell packages pre-compiled with CrossGen
- `Get-Help` content added
- `Get-Help` null reference exception fixed
- Ubuntu 16.04 support added
- Unsupported cmdlets removed from Unix modules
- PSReadLine long prompt bug fixed
- PSReadLine custom key binding bug on Linux fixed
- Default terminal colors now respected
- Semantic Version support added
- `$env:` fixed for case-sensitive variables
- Added JSON config files to hold some settings
- `cd` with no arguments now behaves as `cd ~`
- `ConvertFrom-Json` fixed for multiple lines
- Windows branding removed
- .NET CoreCLR Runtime patched to version 1.0.4
- `Write-Host` with unknown hostname bug fixed
- `powershell` man-page added to package
- `Get-PSDrive` ported to report free space
- Desired State Configuration MOF compilation ported to Linux
- Windows 2012 R2 / Windows 8.1 remoting enabled
