updated tasks to reflect recent changes
updated tasks to reflect recent changes

- removed CAKE and OPTIONS variables
- added PEGJS, MOCHA, MAIN_PARSER and ALT_PARSER variables
- removed 'generator' and 'spec' tasks
- updated 'parser', 'benchmark', 'test' and 'clean' tasks
- added 'build' and 'alt-parser' tasks

erispa/pegx