Setting to specify which request headers to store.

This change allows you to specify which request headers you want to store:

```javascript
var replay = require('replay');
replay.request_headers = [/^authorization/, /^content-type/, /^host/, /^if-/, /^x-/];
```

As I'm currently developing an API wrapper I use replay a lot. The API I'm wrapping is authentication only so with each request I have to send an authorization header. With basic auth as authentication method I don't want to store the authorization header in the replay files because I push those to github for travis ci.