allow setting headers globally in swaggerApi

[rintcius]

this patch enables setting headers like this:

window.swaggerUi = new SwaggerUi({
    discoveryUrl:"http://petstore.swagger.wordnik.com/api/resources.json",
    dom_id:"swagger-ui-container",
    headers: { "Authorization": "XXXX", "someOtherHeader": "YYYY" },
    supportHeaderParams: true,
    supportedSubmitMethods: ['get', 'post', 'put']
});

These headers will then be sent with every request. This is what i use for Basic Auth. so it could also address swagger-api/swagger-ui#53 (although this patch is not tied to the api key)