Minor JSGF header format issue

[Dane Finlay]

Admittedly a minor issue, but I've just noticed the Grammar class and parser don't make the charset and language identifiers in the grammar header optional (see JSGF spec section 3.1). It shouldn't be difficult to change the parser to make these optional and the Grammar class to compile grammar headers with only the values that have been set.

This shouldn't make any difference for compiling grammars for CMU Pocket Sphinx, as it accepts grammar headers as per the spec.