Add properties getters to use gradle.build values

[nartex]

No breaking changes !

If GROUP is not set, use packageName If VERSION_NAME is not set, use build.gradle versionName If VERSION_CODE is not set, use build.gradle versionCode If POM_SCM_URL is not set, use POM_URL If POM_SCM_DEV_CONNECTION is not set, use POM_SCM_CONNECTION If POM_PACKAGING is not set, use "aar"

If required value is not set, say "You must set PROPERTY in gradle.properties file."

[AlexanderLS]

@nartex your commits merged to https://github.com/Vorlonsoft/GradleMavenPush