Fix for issue #11: Use ShadowPlugin to build benchmarks jar

This pull request adds dependency on [ShadowPlugin] (https://github.com/johnrengelman/shadow) and uses it's `ShadowJar` task to build final jar with benchmarks.

What this allows to do is to use full specter of features from the [ShadowPlugin] (https://github.com/johnrengelman/shadow). For example to solve Spring issue (i.e. issue #11) one can simply do the following inside `build.gradle` file:
```groovy
jmhJar {
  append('META-INF/spring.handlers')
  append('META-INF/spring.schemas')
}
```
And this will force `ShadowPlugin` to combine all `META-INF/spring.handlers` files into single file (and the same is true for `spring.schemas`).