Added PA2 regression tester
Explicitly imported packages in miniJava.AbstractSyntaxTrees
Corrected operator-precedence implementation

Precedence climbing was implemented incorrectly in a number of ways:
- Parentheses and UNOPs were handled out of order
- A raw comparison (==) was used instead of String.equals() for token spelling check
- Operators in parseX() methods were checked with IFs, not WHILEs
- Rather than constructing BinaryExprs until no ops were found, it was returned immediately

These bugs have been corrected. All PA2 regression tests now pass.