add and configure JaCoCo maven plugin

In order to get a code coverage report, JaCoCo's [1] maven plugin is
added an properly configured [2].

Steps to generate a code coverage report (from root of this project):
```
mvn clean verify
mvn -f core/pom.xml jacoco:report
```
If the build succeeds point your browser to
`core/target/site/jacoco/index.html`.

[1] http://www.jacoco.org/
[2] http://www.jacoco.org/jacoco/trunk/doc/maven.html

Author: agebhar1

core/pom.xml

@@ -27,6 +27,7 @@
         <projectpath>json-schema</projectpath>
         <maven.compiler.source>1.8</maven.compiler.source>
         <maven.compiler.target>1.8</maven.compiler.target>
+        <jacoco.version>0.7.8</jacoco.version>
     </properties>
     <name>everit-org/json-schema</name>
     <url>https://github.com/everit-org/json-schema</url>
@@ -71,6 +72,30 @@
                     </instructions>
                 </configuration>
             </plugin>
+            <plugin>
+                <groupId>org.jacoco</groupId>
+                <artifactId>jacoco-maven-plugin</artifactId>
+                <version>${jacoco.version}</version>
+                <executions>
+                    <execution>
+                        <id>default-prepare-agent</id>
+                        <goals>
+                            <goal>prepare-agent</goal>
+                        </goals>
+                        <configuration>
+                            <propertyName>surefireArgLine</propertyName>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
+            <plugin>
+                <artifactId>maven-surefire-plugin</artifactId>
+                <groupId>org.apache.maven.plugins</groupId>
+                <version>2.19.1</version>
+                <configuration>
+                    <argLine>${surefireArgLine}</argLine>
+                </configuration>
+            </plugin>
         </plugins>
     </build>
     <dependencies>