Add support for building with Gradle

[rtyler]

I hope this doesn't come across as too forward, I wanted to create this as a separate pull request before I moved onto #27.

I figured I'd take a stab at it, my feelings won't be hurt if the maintainers of the repo wish to stick with Ant though.

[scarytom]

I was actually thinking of moving this project to pure Maven (see #21). What is your interest in using Gradle? This might be a better fit, as we use a lot of Gradle internally at TIM Group, but I thought that Maven was more acceptable in the wild.

[rtyler]

@scarytom My employer does quite a bit of Android development and Gradle is becoming more and more common there. Maven certainly seems to be popular amongst the Java community but I personally find that Gradle is more concise and easy to reason about (that last part is obviously subjective),.

At the end of the day, publishing is going to be to Maven repositories one way or the other, so I'm not sure many end-users are going to care.

[waynr]

https://github.com/DataDog/java-dogstatsd-client appears to be a more actively maintained version of this project, I recommend submitting your patch there if you have not already done so.