Added exec method to Recipe, addressing Issue 1689

Raised NoSuchResourceType if exec is called.  Not sure if this is the best exception to throw, but seemed reasonable to me. 
I don't know if it matters, but I'd expect to see this written as:

```
raise Chef::Exceptions::NoSuchResourceType, "exec was called, but you probably meant to use an execute resource.  If not, please call Kernel#exec explicitly."
```

`Chef::Exceptions::ResourceNotFound` seems more specific to me, as NoSuchResourceType is used in mapping general and platform specific resources to a platform.