taxonomy: make node type a separatw table

I wanted to add caching by a static variable to
taxonomy_get_vocabularies(). While I was doing this, I remembered a
discussion about putting the node types in their own table and extended
the patch to do this.
A problem is that all selects on the vocabulary table will no longer
contain the node types. Does anybody know of where this might cause
problems? taxonomy_get_vocabularies() will also not return the node type
if called without parameter. I solved the only problem I found that
was caused by this.

The patch is incomplete, it does not contain update information and
database schemes. It also isn't tested. I'd like to get some feedback on the implementation.