Fix error on negative GMT timezones.

As @rain2o reported in #16 there is a JavaScript error when using negative timezones (.e.g ``GMT-5``).
The problem is that we support ``/`` as well as ``-`` as separator in the date/time string and the ``GMT-5`` matches a search for ``-`` but shouldn't.

This changes explicitly removes ``GMT-*`` timezone definitions before parsing.
Fixes #16 

@maethu please take a look :wink: