Create a Return Statement.

MiniJava's grammar specifies a return statement as part of the method production. Let's not carry that over in to our AST, however. It should support a generic Return Statement, which closes #7.

StatementReturn is primarily logic factored out of Method.
	src/Compiler/AST/StatementReturn.h
	src/Compiler/AST/StatementReturn.cpp
	src/Compiler/AST/Method.h
	src/Compiler/AST/Method.cpp

Fix AST building, which involves changing main() to return a type int instead of void, so we can return a proper status code to the OS.
	src/Compiler/Language/MiniJava/ContextBuilder.cpp

The changes to code generator end up being pretty trivial.
	src/code_gen.cpp