PowerShell 6 Core Support (#35)

## About
This pull request reflects all changes done in the `linuxsupport` branch.

## Content
- Enable PowerShell 6 Core support
- Use PFX Certificate for encryption ( fixes #32 )
- Updates CI / CD pipeline ( fixes #31 )
- uses portable libressl ( fixes #34 )
- adds `-PassThru` switch for returning current `VIServer` session in `Connect-To` ( fixes #34 )
- adds git lfs for embedded libressl files
- restructured internal functions into `Private` dir
- added certificate related functions
- adds travis build pipeline for tests

src/Private/Get-TempDir.ps1

@@ -0,0 +1,44 @@
+function Get-TempDir {
+    <#
+    .SYNOPSIS
+        Returns the valid temp dir of the current OS
+
+    .DESCRIPTION
+        Returns the valid temp dir of the current OS.
+
+    .INPUTS
+        [None]
+    .OUTPUTS
+        [string]
+
+    .EXAMPLE
+        Get-TempDir
+
+    .NOTES
+        File Name   : Get-TempDir.ps1
+        Author      : Marco Blessing - marco.blessing@googlemail.com
+        Requires    :
+
+    .LINK
+        https://github.com/OCram85/PSCredentialStore
+    #>
+    [CmdletBinding()]
+    [OutputType([string])]
+    param()
+    begin {
+
+    }
+    process {
+        if ($IsLinux) {
+            return (Resolve-Path -Path '/tmp/').Path
+        }
+        if ($IsMacOS) {
+            return (Resolve-Path -Path '/tmp/').Path
+        }
+        elseif (($isWindows) -or ($PSVersionTable.PSVersion.Major -lt 6) -or ($PSVersionTable.PSEdition -eq 'Desktop')) {
+            return (Resolve-Path -Path $env:TEMP).Path
+        }
+    }
+    end {
+    }
+}