v0.3.0 alpha release of Open PowerShell

Supports Windows, Nano, OS X, Ubuntu 14.04, and CentOS 7.1.

With 304 non-merge, non-imported commits since v0.2.0, here are the
major changes:

- .NET Core packages are build rc3-24011
- Native Linux commands are not shadowed by aliases
- `Get-Help -Online` works
- `more` function respects the Linux `$PAGER`; defaults to `less`
- `IsWindows`, `IsLinux`, `IsOSX`, `IsCore` built-in PowerShell variables added
- `Microsoft.PowerShell.Platform` removed for the above
- Cross-platform core host is now `CoreConsoleHost`
- Host now catches exceptions in `--command` scripts
- Host's shell ID changed to `Microsoft.PowerShellCore`
- Modules that use C# assemblies can be loaded
- `New-Item -ItemType SymbolicLink` supports arbitrary targets
- Readline implementation supports multi-line input
- `Ctrl-R` provides incremental reverse history search
- `$Host.UI.RawUI` now supported
- `Ctrl-K` and `Ctrl-Y` for kill and yank implemented
- `Ctrl-L` to clear screen now works
- Documentation was completely overhauled
- Many more Pester and xUnit tests added
