Add initial support for Fuchu tests

Something like this has been discussed at #234, and there is an open
issue for this more specifically at #742. And while there seem to be
some questions of how to do this in the best way, or polish it, I am of
the opinion that it is better to have something than nothing. It is
rather reassuring to n00bs particularly when they see that tools are at
least aware of each other, and this is one of the big draws of FAKE.

The code is derived from the example from Fuchu itself:

    https://github.com/mausch/Fuchu/blob/5ef9ec303abf56f8ca5511cda3fb53bfa705007c/build.fsx#L119

src/app/FakeLib/FakeLib.fsproj

@@ -1,4 +1,4 @@
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="utf-8"?>
 <Project ToolsVersion="4.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
   <PropertyGroup>
@@ -71,6 +71,7 @@
     <Compile Include="FileUtils.fs" />
     <Compile Include="UnitTest\UnitTestCommon.fs" />
     <Compile Include="UnitTest\UnitTestHelper.fs" />
+    <Compile Include="FuchuHelper.fs" />
     <Compile Include="UnitTest\NUnit\Xml.fs" />
     <Compile Include="UnitTest\NUnit\Common.fs" />
     <Compile Include="UnitTest\NUnit\Sequential.fs" />

src/app/FakeLib/FuchuHelper.fs

@@ -0,0 +1,19 @@
+module Fake.FuchuHelper
+
+/// Execute Fuchu tests from one or more assemblies.
+/// Multiple assemblies are run concurrently.
+/// ## Parameters
+/// 
+///  - `testExes` - The paths of the executables containing Fuchu tests to run.
+let Fuchu testExes = 
+    let errorCode =
+        testExes
+        |> Seq.map (fun program -> if not isMono
+                                   then program, null
+                                   else "mono", program)
+        |> Seq.map (fun (program, args) -> asyncShellExec { defaultParams with Program = program; CommandLine = args })
+        |> Async.Parallel
+        |> Async.RunSynchronously
+        |> Array.sum
+    if errorCode <> 0
+    then failwith "Unit tests failed"