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-ModuleBase.ps1

@@ -0,0 +1,29 @@
+function Get-ModuleBase {
+    <#
+    .SYNOPSIS
+        Returns the base path of the current module.
+
+    .DESCRIPTION
+        This is just a wrapper for enabling pester tests.
+
+
+    .OUTPUTS
+        Returns the base path as string
+
+    .NOTES
+        File Name   : Get-ModuleBase.ps1
+        Author      : Marco Blessing - marco.blessing@googlemail.com
+        Requires    :
+
+    .LINK
+        https://github.com/OCram85/PSCredentialStore
+#>
+    [CmdletBinding()]
+    [OutputType()]
+    param()
+    begin {}
+    process {
+        return $MyInvocation.MyCommand.Module.ModuleBase
+    }
+    end {}
+}