Code refactoring: relativity_query becomes optional module

Looking to the future, I refactored the existing relativity.module into two parts:

- relativity.module, which deals with linking nodes and displaying those links
- relativity_query.module, which implements a new content type that is then used to navigate and search the link structure defined by relativity module.

There are several reasons for this separation:
- 99% of people use only relativity linking and displaying functionality. relativity_query.module is now optional.
- it is much easier to understand and follow the code now (important issue, since javanaut, the original author, no longer maintains the module).
- relativity_query will be easier to modify to take advantage of new node relationship APIs.

From the user perspective, there should be no changes (apart from enabling relativity_query module, if necessary). I am attaching the two new files. If someone could test this, I'd appreciate any comments, reports, etc. If I don't hear anything, I will check these in in a few weeks to CVS HEAD. The 4.7 version will remain as a stable release.

Darius