Register EXSLT namespaces by default (resolves #470)

`lxml` supports interesting [EXSLT](http://www.exslt.org/) extensions to XPath 1.0 (via `libxslt`).
This PR register some of the corresponding namespaces by default in the `Selector` class.

The most useful IMO are:
* [set manipulation](http://www.exslt.org/set/index.html), using prefix `set:` (namespace `http://exslt.org/sets`)
* [regular expressions](http://www.exslt.org/regexp/index.html), and especially `regexp:test()` when one needs to use regex inside a XPath; using prefix `regexp:` (namespace `http://exslt.org/regular-expressions`)

One could add `re:` as an alias prefix for regular expressions.
(Regex support is in fact NOT in `libxslt` but is implemented in `lxml` using XPath extensions in Python)

Only regex tests added for now.