Representation of queries for the database is now abstract

src/qir/ast/QIRDBNode.java

src/qir/ast/data/QIRTable.java

src/qir/driver/DBDriver.java

src/qir/driver/OracleDriver.java

src/qir/driver/PostgreSQLDriver.java

src/qir/driver/SQLDriver.java

src/qir/driver/SQLStringDriver.java

src/qir/visitor/IQIRVisitor.java

src/qir/visitor/QIRCompileVisitor.java

src/qir/visitor/QIREvaluationVisitor.java

src/qir/visitor/QIRFreeVarsVisitor.java

src/qir/visitor/QIRGreedyReduceVisitor.java

src/qir/visitor/QIRSQLVisitor.java