Added per-level empty maps to the hierarchical maps in ConstraintSolver

This solves a NPE if the definitions file is completely empty
Merge branch 'working' of github.com:gurupras/worldtree into working
Updated Parser to accept standalone query statements

Due to recent changes in the way query level was handled, the Parser
could no longer be used to run tests such as those in InitializationTest
Updated InitializationTest to use world.properties rather than init.properties
Updated QueryResolutionEngine to allow joins on both lhs and rhs

This commit needs further testing. Only a few simple queries have been
tested so far.