add dirtyGeokrety

Breaking change : rename Gkm\Domain\GeoKrety to Gkm\Domain\Geokrety

GkmClient
- Fix #11 add dirtyGeokretyById
- Fix #12 add asyncDirtyGeokretyById

add UsageSample
update readme about requirements
improve code coverage target

Travis set 7.3 again https://travis-ci.community/t/please-add-php-7-3-on-xenial/1238/2

add README 3 badges build,release,download

- using Torann/json-ld as example

composer require geokretymap/php-client