Javadoc & import clean up (#20)

- replace full class name with import
- use modern javadoc tags

src/test/groovy/HelloSpockSpec.groovy

@@ -14,7 +14,9 @@
  * limitations under the License.
  */
 
-class HelloSpockSpec extends spock.lang.Specification {
+import spock.lang.Specification
+
+class HelloSpockSpec extends Specification {
   def "length of Spock's and his friends' names"() {
     expect:
     name.size() == length

src/test/groovy/StepwiseExtensionSpec.groovy

@@ -15,13 +15,13 @@
 import spock.lang.*
 
 /**
- * Demonstrates how @Stepwise causes a spec to be run in incremental steps.
- * Change a step's condition from <tt>true</tt> to <tt>false</tt>, and observe
+ * Demonstrates how {@code @Stepwise} causes a spec to be run in incremental steps.
+ * Change a step's condition from {@literal true} to {@literal false}, and observe
  * how the remaining steps will be skipped automatically on the next run.
  * Also notice that if you run a single step (e.g. from the IDE's context menu),
  * all prior steps will also be run.
  *
- * <p><tt>&#64;Stepwise</tt> is particularly useful for higher-level specs whose
+ * <p>{@code @Stepwise} is particularly useful for higher-level specs whose
  * methods have logical dependencies.
  *
  * @since 0.4