Add gradle plugin to flyway project

A gradle plugin based on the Flyway 2.x series
([original](https://github.com/ben-manes/gradle-flyway-plugin)).

This plugin is integrated into the Maven build using the
[gradle-maven-plugin](https://github.com/if6was9/gradle-maven-plugin)
to invoke the gradle build. The gradle build self-installs using the
wrapper and, on an install, publishes to the local maven repository.
This approach was chosen as the gradle apis are not published to a
maven repository and that dependency appears to have blocked previous
efforts for integrating a plugin into flyway.

Due to the multi-build integration, I am unsure whether `mvn deploy`
will recognize the produced artifact and attach it during a
`mvn release:prepare release:perform` process. This should be tested by
the project owner.

A sample project is included. A richer
[sample](https://github.com/ben-manes/gradle-jooq-plugin/tree/master/example)
shows integration of flyway, H2, jOOQ, and ModelMapper for performing
round-trip operations in a test class.

A known limitation is that the plugin does not pick up Java-based
migrations. This is likely due to the class files not being on the
classpath, as the `buildscript` is setup prior to the project. This may
require invoking flyway as a JavaExec or command-line process instead.
As Gradle does not promote itself as a deployment tool (see fabric,
capistrano, etc) the SQL migrations may suffice for the community's
needs.