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-RandomAESKey.ps1

@@ -0,0 +1,44 @@
+function Get-RandomAESKey {
+    <#
+    .SYNOPSIS
+        Generate a new 32-byte AES key.
+
+    .DESCRIPTION
+        Uses the System.Security.Cryptography namespace for random aes key generation.
+
+    .INPUTS
+        [None]
+
+    .OUTPUTS
+        [byte[]]
+
+    .EXAMPLE
+        .\Get-RandomAESKey
+
+    .NOTES
+        File Name   : Get-RandomAESKey.ps1
+        Author      : Marco Blessing - marco.blessing@googlemail.com
+        Requires    :
+
+    .LINK
+        https://github.com/OCram85/PSCredentialStore
+    #>
+
+    [CmdletBinding()]
+    [OutputType([byte[]])]
+    param()
+
+    begin {}
+
+    process {
+        $key = [byte[]]::new(32)
+        $rng = [System.Security.Cryptography.RNGCryptoServiceProvider]::Create()
+        $rng.GetBytes($key)
+        Write-Output $key
+        if ($null -ne $key) {
+            [array]::Clear($key, 0, $key.Length)
+        }
+
+    }
+    end {}
+}