Small fix to "null"-type's assignment rules
Small fix to main's "String[]" dummy type
Fixed and improved NameAnalysis

fix: visit(Method) did enterScope but not leaveScope!
fix: LocalVarDeclStmtInit did not add variable to SymbolTable
add: throw if this is not found (...expected to happen in main)
add: tests
Implemented recursive AST-node visitor base-type
Fixed parser: use "this" for context-less method-calls
Added return statement validator
Refactored type analysis (using recursive AST-node visitor base)
Added lots of tests