Migrate plugin conventions to extensions

[x] All tests passed. If this feature is not already covered by the tests, I added new tests.

Fix https://github.com/jfrog/artifactory-gradle-plugin/issues/43 Supersedes https://github.com/jfrog/artifactory-gradle-plugin/pull/62

The notion of Gradle conventions has been deprecated. In essence, this pull request is intended migrate of the plugin to the extensions concept by eliminating all occurrences of ConfigureUtil.configureUsing(Closure closure) and replacing them with ObjectFactory.newInstance(Class, Object...). It also replaces all references to ConventionUtils with ExtensionsUtils.

As per the information provided in the ConfigureUtil documentation:

Gradle automatically generates a Closure-taking method at runtime for each method with an Action as a single argument as long as the object is created with ObjectFactory.newInstance(Class, Object...).

To utilize ObjectFactory.newInstance, a constructor must have the @Inject annotation applied.

jfrog / artifactory-gradle-plugin

Migrate plugin conventions to extensions #72