Add Project Identifier variable and target
* A variable (IDENTIFIER) was added to allow the 'identifier' target to search source code and header files for a matching hash parametrically. * The phony 'identifier' target greps for the IDENTIFIER pattern in each file and fails if any source/header doesn't contain the pattern. * The identifier target is added as a dependency to fullsubmit and partialsubmit. * Editing IDENTIFIER is now part of the first student TODO in the Makefile.