Pipeline folders (#96)

* Move XamlBuild funcs to new Build top-level folder

* Add Get-BuildFolder

* Add New-BuildFolder support

* Refactor Get-BuildFolder

* Fix missing error message

* Add Remove-BuildFolder support

* Add Microsoft.VSS.Release.Client Nuget package

* Add Get-ReleaseDefinitionFolder support

* Rename funcs to *BuildDefinitionFolder

* Add release folder management funcs

* Update release notes

* Remove dependency on PSYaml

Package apparently is deprecated

* Add PowerShell-Yaml explicitly to the build

Build.ps1

@@ -17,7 +17,7 @@ Param
 Function Install-Dependencies
 {
     $NugetPackages = @('GitVersion.CommandLine')
-    $PsModules = @('psake', 'PsScriptAnalyzer', 'VSSetup', 'PSYaml', 'Ps1xmlgen')
+    $PsModules = @('psake', 'PsScriptAnalyzer', 'VSSetup', 'powershell-yaml', 'ps1xmlgen')
 
     $script:PackagesDir = Join-Path $SolutionDir 'packages'