Add Gradle Wrapper with version 1.7 of Gradle.

This makes it easier to create reproducible builds of the plugin.

Resolves #14.
Remove usage of deprecated property

Resolves #15.
Remove usage of deprecated method

Resolves #15.
No need for groovy configuration

The dependency on groovy need not be specified through the groovy
configuration anymore, a regular compile dependency will suffice.

Upgrade groovy at the same time (1.8.4 --> 1.8.6).

Resolves #15.
Our own properties must have ext. in front

Resolves #15.
Use temporary versioning logic until plugin is ok

The release plugin does not work under the newer releases of Gradle,
so use 'git describe' for setting the version. This command will take
the version from the current tag.