Add the Exec Maven Plugin

This allows execution of the Codegen class via `mvn exec:java`,
with optional args passed via `-Dexec.args="..."`:

mvn exec:java -Dexec.args="-i .../swagger.json -l html -o samples/..."

pom.xml

@@ -68,6 +68,21 @@
     <directory>target</directory>
     <finalName>${project.artifactId}-${project.version}</finalName>
     <plugins>
+      <plugin>
+        <groupId>org.codehaus.mojo</groupId>
+        <artifactId>exec-maven-plugin</artifactId>
+        <version>1.3.2</version>
+        <executions>
+          <execution>
+            <goals>
+              <goal>java</goal>
+            </goals>
+          </execution>
+        </executions>
+        <configuration>
+          <mainClass>com.wordnik.swagger.codegen.Codegen</mainClass>
+        </configuration>
+      </plugin>
       <plugin>
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-jar-plugin</artifactId>