Polish
Apply source cleanup and formatting
Add mainClass option for Repackage task

Add `mainClass` property option to the Repackage task. If the property
is defined within a task, it works in the same way as if it defined
within the springBoot{} 'ext' properties section.

Option is valid only for that specific task where it is defined, and
will override option defined in springBoot{} ext properties.

Fixes gh-283
Merge pull request #302 from jvalkeal/gradle-mainclass

* gradle-mainclass:
  Add mainClass option for Repackage task
Polish gradle repackage task formatting
Update gradle example builds