test: add phpunit configuration file

Tests can now be run with just `vendor/bin/phpunit`.
test: update travis settings
doc: correct stated minimum PHP version

close #4
refactor: change service class to call toRequestXml method

Service and request object classes have to be refactored due to changes
in base classes.

[3d60a5e](https://github.com/davidtsadler/ebay-sdk/commit/3d60a5e29ff21edf971c1bcfea9efb96275b688b)
refactor: pass request object to callOperation method

The method parameters of BaseService::callOperation has changed so that
a request object must be passed is as the second parameter.

[27bff98](https://github.com/davidtsadler/ebay-sdk/commit/27bff98383695381e6872430ae91b495be9a9b4d)
feature: allow auth token to be passed as configuration option

Auth tokens can now be passed as a configuration option of the
TradingService class.

```php
use \DTS\eBaySDK\Trading\Services;

$service = new Services\TradingService(array(
    'authToken' => 'YOUR_PRODUCTION_USER_TOKEN_APPLICATION_KEY'
));
```

Note that the TradingServce will *inject* the auth token into
request objects when calling the API. It will do this by setting the
correct properties on the request object. This will mean that the
service will modify your request objects.

```php
use \DTS\eBaySDK\Trading\Services;
use \DTS\eBaySDK\Trading\Types;

$service = new Services\TradingService(array(
    'authToken' => 'YOUR_PRODUCTION_USER_TOKEN_APPLICATION_KEY'
));

$request = new Types\GeteBayOfficialTimeRequestType();
$assert(null, request->RequesterCredentials);

/**
 * This will modify the request by adding the auth token.
 */
$response = $service->geteBayOfficialTime($request);

/**
 * The auth token will match the one set in the configuration.
 */
assert('YOUR_PRODUCTION_USER_TOKEN_APPLICATION_KEY' === $request->RequesterCredentials->eBayAuthToken);
```

You can continue to set auth tokens in the request as the request will
take precedence over the configuration.

```php
use \DTS\eBaySDK\Trading\Services;
use \DTS\eBaySDK\Trading\Types;

$service = new Services\TradingService(array(
    'authToken' => 'YOUR_PRODUCTION_USER_TOKEN_APPLICATION_KEY'
));

$request = new Types\GeteBayOfficialTimeRequestType();
$request->RequesterCredentials = new Types\CustomSecurityHeaderType();
$request->RequesterCredentials->eBayAuthToken = 'TOKEN_IN_REQUEST';

/**
 * This will **NOT** modify the request as the auth token in the request
 * takes precendence.
 */
$response = $service->geteBayOfficialTime($request);

assert('TOKEN_IN_REQUEST' === $request->RequesterCredentials->eBayAuthToken);
```
doc: update CHANGELOG