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

resources/cs/Broken_CS.json

@@ -1,3 +1,3 @@
 {
-    "Version":  "1.2.0",
-    "Creation":  "2016-06-14 08:41:10"
+    "Version": "2.0.0",
+    "Creation": "2016-06-14 08:41:10"

resources/cs/Challenge.bin

@@ -1 +0,0 @@
-!<21><><EFBFBD>p<EFBFBD><07><>H4<48><34><EFBFBD>"=w<><77>S<EFBFBD>2