Added option to pass jvmArgs.

Patch for issue #46. This allows for passing jvmArgs through the test extension, in the spirit of http://www.gradle.org/docs/current/javadoc/org/gradle/process/JavaForkOptions.html#jvmArgs(java.lang.Object...)

```
robolectric {
   // Increase MaxPermSize
   jvmArgs '-XX:MaxPermSize=512m'
}
```

This is my first Groovy contribution, so I apologize if anything is weird :)