Parser:
1. add group by and having support
2. builtin functions (at which level should functions be resolved?
(parser level or execution level?)

Execution:
1. create a QueryResult class to hold the query result
2. implement query execution for simple queries in NXJCRConnector

API:
1. link IRepository/IWorkspace with the query module
(add a public QueryResult executeQuery(Query query) method in IRepository or IWorkspace)
