Let extended translators override what XPathExpr class is used

GenericTranslator offers an excelent way to support custom selectors
trough method hooks and allowing to return a *new* XPathExpr from this
hooks.

The main problem is that returning extended `XPathExpr` instances fail
for combiners because `XPathExpr.join()` assume a fixed XPathExpr
instance attributes (element, path and condition) to copy from `other` to `self`

`XPathExpr.join()` can be extended in subclass but needs that `left`
xpath instance to be of the extended class too, and right now we can
only control `right` xpath type.

The problem can be mitigated by recasting all xpath returned from
`GenericTranslator.xpath_element()` that only works because it is the
only hook that cast `XPathExpr` instances.

The proposed change allow projects extending GenericTranslator to also
safely extend `XPathExpr` to correctly support combiners in extended
features.
Merge pull request #22 from dangra/xpathexpr_cls

Let extended translators override what XPathExpr class is used