Added IpInfoDB country precision endpoint

This PR allows to use the country precision endpoint of IpInfoDB (http://ipinfodb.com/ip_location_api.php).
It's said to be faster and recommended when not needing the city precision.

Also, this PR fixes interfaces from http-message in the tests.
For example, the `Psr\Http\Message\ResponseInterface` does not exist in`psr/http-message ~0.5.0` which is required by `egeloen/http-adapter ~0.1` which is required by `willdurand/geocoder dev-master`: https://github.com/php-fig/http-message/tree/0.5.1/src

In fact the testsuite failed on a fresh install with:
``` console
$ phpunit                                                           [127]
PHPUnit 4.3.5 by Sebastian Bergmann.

Configuration read from /home/gildas/projects/Geocoder/phpunit.xml.dist

...........................................PHP Fatal error:  Call to undefined method Mock_ResponseInterface_d5ae35c4::getBody() in /home/gildas/projects/Geocoder/src/Geocoder/Provider/ArcGISOnline.php on line 176
PHP Stack trace:
PHP   1. {main}() /home/gildas/.composer/vendor/phpunit/phpunit/phpunit:0
PHP   2. PHPUnit_TextUI_Command::main() /home/gildas/.composer/vendor/phpunit/phpunit/phpunit:56
PHP   3. PHPUnit_TextUI_Command->run() /home/gildas/.composer/vendor/phpunit/phpunit/src/TextUI/Command.php:138
PHP   4. PHPUnit_TextUI_TestRunner->doRun() /home/gildas/.composer/vendor/phpunit/phpunit/src/TextUI/Command.php:186
PHP   5. PHPUnit_Framework_TestSuite->run() /home/gildas/.composer/vendor/phpunit/phpunit/src/TextUI/TestRunner.php:423
PHP   6. PHPUnit_Framework_TestSuite->run() /home/gildas/.composer/vendor/phpunit/phpunit/src/Framework/TestSuite.php:751
PHP   7. PHPUnit_Framework_TestCase->run() /home/gildas/.composer/vendor/phpunit/phpunit/src/Framework/TestSuite.php:751
PHP   8. PHPUnit_Framework_TestResult->run() /home/gildas/.composer/vendor/phpunit/phpunit/src/Framework/TestCase.php:711
PHP   9. PHPUnit_Framework_TestCase->runBare() /home/gildas/.composer/vendor/phpunit/phpunit/src/Framework/TestResult.php:643
PHP  10. PHPUnit_Framework_TestCase->runTest() /home/gildas/.composer/vendor/phpunit/phpunit/src/Framework/TestCase.php:775
PHP  11. ReflectionMethod->invokeArgs() /home/gildas/.composer/vendor/phpunit/phpunit/src/Framework/TestCase.php:905
PHP  12. Geocoder\Tests\Provider\ArcGISOnlineTest->testGetGeocodedDataWithInvalidData() /home/gildas/.composer/vendor/phpunit/phpunit/src/Framework/TestCase.php:905
PHP  13. Geocoder\Provider\ArcGISOnline->getGeocodedData() /home/gildas/projects/Geocoder/tests/Geocoder/Tests/Provider/ArcGISOnlineTest.php:22
PHP  14. Geocoder\Provider\ArcGISOnline->executeQuery() /home/gildas/projects/Geocoder/src/Geocoder/Provider/ArcGISOnline.php:71
```
Are you ok with this fix @willdurand? Otherwise the test fails locally