#6 better support for simpleType definitions.

[Kai Runte]

Several of the built-in simple types are missing (c.f.
<http://www.w3.org/TR/xmlschema-2/#built-in-datatypes>),
but more importantly, Pedro is missing some mechanisms
for extending simpleTypes. So far, only restrictions by
regular expressions is implemented, missing are:
-union of two or more simple types
-space separated lists of a base type
-min/max inclusive/exclusive definitions.

It might be useful to have a look at MSV and see if we
can re-use some of its validator in Pedro directly...