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/Test-Module.ps1

@@ -0,0 +1,79 @@
+function Test-Module {
+    <#
+    .SYNOPSIS
+        Tests if the given module exists on the local system.
+
+    .DESCRIPTION
+        Tests if the given module is installed on the local system. It returns a bool value as result.
+
+    .PARAMETER Name
+        Define a item name you need to test
+
+    .PARAMETER Type
+        Define the dependency type. This could be a Module or PSnapin.
+
+    .PARAMETER MessagePattern
+        You an optionally adjust the message pattern for the error message itself.
+        The available placeholders are:
+        - {0} : Type
+        - {1} : Name
+
+    .PARAMETER StopIfFails
+        This switch forces the entire script to stop if the given dependency object fails.
+
+    .INPUTS
+        [None]
+
+    .OUTPUTS
+        [Bool]
+
+    .EXAMPLE
+        .\Test-Dependency -Name 'VMware.PowerCLI' -Type 'Module'
+
+    .EXAMPLE
+        .\Test-Dependency -Name 'VMware.PowerCLI' -Type 'Module' -StopIfFails
+
+    .NOTES
+        ```
+        File Name   : Test-Module.ps1
+        Author      : Marco Blessing - marco.blessing@googlemail.com
+        Requires    :
+        ```
+
+    .LINK
+        https://github.com/OCram85/PSCredentialStore
+    #>
+    [OutputType([bool])]
+    [CmdletBinding()]
+    param(
+        [Parameter(Mandatory = $true)]
+        [ValidateNotNullOrEmpty()]
+        [string]$Name,
+
+        [Parameter(Mandatory = $false)]
+        [ValidateNotNullOrEmpty()]
+        [string]$MessagePattern = @"
+Could not find the required {0} called {1}. Please install the required {0} to run this function!
+"@,
+        [Parameter(Mandatory = $false)]
+        [switch]$StopIfFails
+    )
+    begin {}
+
+    process {
+        $Message = $MessagePattern -f $Type, $Name
+        Write-Debug $Message
+
+        if (Get-Module -Name $Name -ListAvailable) {
+            return $true
+        }
+        else {
+            if ($StopIfFails) {
+                Write-Error -Message $Message -ErrorAction Stop -Category NotInstalled
+            }
+            return $false
+        }
+    }
+
+    end {}
+}