New submodule Exceptionless to use options rather than exceptions.

I have added an ExceptionLess module (purpose similar to http://ocaml-batteries-team.github.io/batteries-included/hdoc2/BatList.Exceptionless.html for example) with the same functions as in the lib, but that return an `'a option` so that they return `None` instead of raising an exception if they do not find a node for instance.

So, to use these functions, one must do:
```ocaml
open Ezxml.ExceptionLess
```
which will overwrite the versions of these functions that use exceptions.