Update OAuth/ResourceOwner/AbstractResourceOwner.php

Always set `User-Agent` header.
Merge pull request #192 from stloyd/patch-1

Always set `User-Agent` header.
Refactoring of OAuth responses and custom resource owner options

Removed `AdvancedPathUserResponse` & `AdvancedUserResponseInterface`
and merged theirs code into `PathUserResponse` & `UserResponseInterface`.

Now `PathUserResponse` contains by default all common paths keys.

`PathUserResponse#getPath()` & `PathUserResponse#getValueForPath()` no
longer throw an exception when path or value not exists, now they simply
return `null` instead.

Add ability to set custom options for particular resource owners
in by defining them in `config.yml`:

hwi_oauth:
    resource_owners:
        foursquare:
            type:           foursquare
            client_id:      <client_id>
            client_secret:  <client_secret>
            options:
                version:    20121212
Add notes to CHANGELOG file