Added option for XHR response cache control

As per issue #195 added an option to control XHR response caching in the browser.

When set with `cache: false` a random string is append to the query string of the request URL forcing the browser to load the source from the serve on each request.

Default value is `cache: true` which allows the browser to cache responses.

    var ds = new Miso.Dataset({
      url: 'data.csv',
      cache: false,
      delimiter: ','
    });

Includes some rudimentary tests. Ensured all existing tests pass.

Have tested manually in IE8 (resolving the identified issue), Chrome and Firefox.