Add gradle support

[jneira]

• If possible we'll try to keep etlas installation
• Using the eta plugin
• Not sure if a special war goal could be added in this project

[jneira]

@rahulmutt do you think would be possible (and desirable?) the coexistence of both install ways?

[rahulmutt]

You can definitely co-exist with Gradle & Etlas configurations. For Gradle, you just need to configure the sourceSets to point to the source files so it knows where to look. You can take a look at this example.