add opportunity to use properties of project in user and key of bintray publication

publish.gradle

@@ -5,8 +5,8 @@ ext {
 }
 
 bintray {
-    user = System.getenv('BINTRAY_USER')
-    key = System.getenv('BINTRAY_KEY')
+    user = project.hasProperty('BINTRAY_USER') ? project.property('BINTRAY_USER') : System.getenv('BINTRAY_USER')
+    key = project.hasProperty('BINTRAY_KEY') ? project.property('BINTRAY_KEY') : System.getenv('BINTRAY_KEY')
     publications = ["maven"]
     filesSpec {
         into "$projectBintrayDir"