Add a way to get valuespaces

[tbassetto]

For introspection purposes and also because different devices may have slightly different values possible for the same command, being able to get the valuespaces would be really awesome.

[myme]

I have done some work on this already in order to add a completion engine. Will see if it can easily be extracted and generalized.

[myme]

With #62 it will be possible to fetch the schemas/documents from the devices and from that read out valuespaces. It's a half-way solution to this, if what's requested is something like:

xapi.config.SystemUnit.Name.valuespace()