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

tests/Private/01_Get-TempDir.Tests.ps1

@@ -0,0 +1,27 @@
+Describe "Get-TempDir" {
+    Context "Basic tests" {
+        It "Should not throw" {
+            {Get-TempDir} | Should -Not -Throw
+        }
+        It "Should return the correct os tmp path" {
+            $Path = Get-TempDir
+
+            if ($ENV:TRAVIS) {
+                if ($IsLinux) {
+                    $RefPath = (Resolve-Path -Path '/tmp/').Path
+                    $Path | Should -Be $RefPath
+                }
+                if ($IsMacOS) {
+                    $RefPath = (Resolve-Path -Path '/tmp/').Path
+                    $Path | Should -Be $RefPath
+                }
+            }
+            if ($Env:APPVEYOR) {
+                if (($isWindows) -or ($PSVersionTable.PSVersion.Major -lt 6) -or ($PSVersionTable.PSEdition -eq 'Desktop')) {
+                    $RefPath = (Resolve-Path -Path $env:TEMP).Path
+                    $Path | Should -Be $RefPath
+                }
+            }
+        }
+    }
+}