Polishing: consistent indenting in DependencyManagementPluginSpec
Provide concise DSL for dependencies with same group and version

This commit updates the DSL to provide a more concise way to declare
dependency management for multiple dependencies with the same group
and version. Prior to this commit, the group and version had to be
repeated for each dependency:

dependencyManagement {
     dependencies {
          'org.slf4j:slf4j-api' '1.7.7'
          'org.slf4j:slf4j-simple' '1.7.7'
     }
}

This commit introduces a dependency set which means that the group and
version only need to be declared once:

dependencyManagement {
    dependencies {
        dependencySet(group: 'org.slf4j', version: '1.7.7') {
			entry 'slf4j-api'
			entry 'slf4j-simple'
		}
	}
}

Closes #5