Keep the gradient flowing

can we merge now, pleeease ?

Three months after I began to write sympy.queries, I feel it's about time to include it in sympy's trunk, so today I sent for review 4 patches that implement the complete query module. It's been a lot of fun, but it has also caused me some headaches ... specially last month trying to have the code as much bug-free as possible. Next steps is to improve performace of query() and write the refine module. PD: This weekend I'll be at Leipzig for EuroSciPy, hope to meet there some SymPy developers!