Fixed comparison type check.
Added tests for typecheck.
Updated todo.
Renamed left-hand side and right-hand side of Assignment node.
Evaluate Context now maintains a named list of variables.
Updated evaluate value classes.
Updated grammar and parser by adding vardecl and assignment to statement production.
Using better naming convention for readability.
Updated printvisitor for assignments.
Updated typecheck test.
Merge branch 'master' into martyn82