added request headers matching

Added the ability to match request headers.

I'm in the middle of testing some oauth2 services and I needed an easy way to create mocks which would be valid only if an ```Authorization``` header was sent in the request.

This pull requests adds a new property you can define in the mockjax options object called ```requestHeaders```. 
```requestHeaders``` is just an object with key, value pairs defining which headers are needed by the request to be validated.

here is an example which will match all the requests to /me only if the request includes an header called ```Authorization``` with value ```"Bearer vsdfjsdf+adfvadfv="```
```
$.mockjax({
    url: "/me",
    requestHeaders: {
        Authorization: "Bearer vsdfjsdf+adfvadfv="
    },
    responseText: "YEAH!"
}
```
We should probably use jQuery's `$.each(handler.requestHeaders, function(key, value) { ... });` utility here, otherwise we might iterate over values we do not want.