Add support for reloading resources in Gradle plugin

Usage:

$ gradle bootRun
...

Edit files in src/main/resources and see the changes live in a web app
(e.g. static resources in /static).

The old functionality of bootRun has been replaced (since it didn't add
a lot of value and also didn't expose any JMV argument setters of anything).
This new feature set is backed by any existing "run" task configuration.

In addition autodetects a main class if there is one in the project
sources, so no need for mainClassName = '...' in build.gradle.

Applies the 'application' plugin (so no need to declare that either).

Fixes gh-225