Added Travis-CI.

This builds JNA native parts on Linux. Starting small :) We get pull request builds for free to catch simple stuff.

Successful build [here](https://travis-ci.org/dblock/jna/builds/28440930).

@twall, you need to go to https://travis-ci.org/profile/twall and enable building JNA (flip the switch) *before* merging this, please.