added request headers matcher

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!"
}
```
What's the motivation for having both of these checks? If the request header does not exist, won't `v !== value` catch that?