schema / namespaces support

Hi Marijn, 

these changes add a simple macro `with-schema` and some utility functions for creating, droping and listing namespaces to postmodern. They also fix a small bug in table-existing-p and the function searches for tables in the current namespace search path. 

The macro is covered by a simple smoke test and ist documented with all its options. 

I would like to see this addition in postmodern. Feel free to pick or comment; 
this is one of my first public CL hacks. 

Greetings