Added the auto-value plugin stub.

The auto-value plugin and its extension has been created. The extension provides access to the plugins configuration and holds some default values. The plugin already provides a task stub that will generate the auto-value sources directory. The plugin sets up the core java plugin and makes the 'compileJava' task depend on the generation of the auto-value sources.
Minor tweaks to the task definitions in the auto-value plugin.
The auto-value plugin now creates the source folders for the configured generated sources set.
The auto-value plugin now generates the auto-value source files at the configured location.
The auto-value plugin is now functional complete.
Upgraded to Gradle 2.2.1.
Upgraded Gradle wrapper to 2.2.1.
Added GDoc.
Changed default generated sources dir of auto-value plugin to 'src/generated/auto-value'.
Added auto-value plugin section to the README.