Add subgrouping of plugin tasks either by variant name or sub task based on a setting

tasks of the plugin will now subgrouped by default. There are two options to do that:

![command_list_grouped](https://cloud.githubusercontent.com/assets/45467/3016211/744e6e94-df6b-11e3-8ab1-22b8d7084bb7.png)

The default is group by variantName which can be changed to group by sub tasks using the property "sortBySubtasks" (see sample/build.gradle)