Added in(x::String, y::String) = contains(y, x)

I'm not optimistic that this will fly, but I figured I would try. ;-)

If this patch isn't acceptable, we need 

```julia
in(::String, ::String) = error("Use contains(x,y) for string containment")
```

since 
```julia
julia> "not" in "knot"
false
```
says that "not" is not in "knot", which is clearly misleading. :-)

Other than this, to quote Stefan, I'm unreasonably happy about the infix version of "in". ;-)