Add a custom task in .NET 4.0

Hi,

I've added a new custom task in /Samples/CustomTask40 in order to demonstrate the issue that I encontered when I tried to load my custom task in .NET 4.0 in a FAKE file.

Here is the message showed by FAKE.exe when you try to run Samples/CustomTask40/example/build.fsx : 

```
D:\Invenietis\Dev\Web\FAKE\Samples\CustomTask40\example>tools\FAKE\tools\FAKE.exe build.fsx

build.fsx(3,1): warning FS0217: The referenced or default base CLI library 'mscorlib' is 
binary-incompatible with the referenced library 
'D:\Invenietis\Dev\Web\FAKE\Samples\CustomTask40\example\MyCustomTask.dll'. 
Consider recompiling the library or making an explicit reference to a version 
of this library that matches the CLI version you are using.

error FS0193: internal error: Impossible de charger le fichier ou l'assembly
'file:///D:\Invenietis\Dev\Web\FAKE\Samples\CustomTask40\example\MyCustomTask.dll' 
ou une de ses dépendances. Cet assembly a été construit par un runtime plus 
récent que le runtime actuellement chargé et ne peut pas être chargé.
```

Is it possible to do something to allow FAKE to use custom task built with another Framework than FAKE itself ?

Thank you :)