modularize build.gradle

build.gradle has become quite monolithic. This splits out components of the build into separate files using apply from: 

* tasks related to dist
* tasks related to docs 
* tasks related to building modules and extensions
* tasks related to hadoop distros

This is a first cut. I expect this can be further refined.
This is significant: http://forums.gradle.org/gradle/topics/how_do_i_apply_a_plugin_to_a_project_from_a_shared_applied_gradle_file