Add dotnet helpers to check for version and if CLI is installed

src/app/FakeLib/DotNetCLIHelper.fs

@@ -1,10 +1,32 @@
-/// Contains a task which can be used to run [DotCover](http://www.jetbrains.com/dotcover/) on .NET assemblies.
+/// Contains a task which can be used to run dotnet CLI commands.
 module Fake.DotNet
 
 open Fake
 open System
 open System.Text
 
+let commandName = "dotnet"
+
+/// Gets the installed dotnet version
+let getVersion() = 
+    let processResult = 
+        ExecProcessAndReturnMessages (fun info ->  
+          info.FileName <- commandName
+          info.WorkingDirectory <- Environment.CurrentDirectory
+          info.Arguments <- "--version") (TimeSpan.FromMinutes 30.)
+
+    processResult.Messages |> separated ""
+
+/// Checks wether the dotnet CLI is installed
+let isInstalled() =
+    let processResult = 
+        ExecProcessAndReturnMessages (fun info ->  
+          info.FileName <- commandName
+          info.WorkingDirectory <- Environment.CurrentDirectory
+          info.Arguments <- "--version") (TimeSpan.FromMinutes 30.)
+
+    processResult.OK
+
 /// DotNet Restore parameters
 type RestoreParams = {
     /// ToolPath - usually just "dotnet"
@@ -21,7 +43,7 @@ type RestoreParams = {
 }
 
 let private DefaultRestoreParams : RestoreParams = {
-    ToolPath = "dotnet"
+    ToolPath = commandName
     WorkingDir = Environment.CurrentDirectory
     NoCache = false
     TimeOut = TimeSpan.FromMinutes 30.