Add 'findbugs' support in to this project

[maxiaohao]

No description provided.

[maxiaohao]

As v1.0 released, there will be more developer contribute their code to aws-mock, thus we need to pick this enhancement task and have it done within 1 month for automated code quality check on each pull request.

[nottoseethesun]

How will this differ from the Travis continuous build that we already have going?

[maxiaohao]

Here 'findbugs' means: http://findbugs.sourceforge.net/ - the tool which works much like our existing checkstyle rules for code quality assurance. Like checkstyle, we can do the findbugs check (http://www.gradle.org/docs/current/userguide/findbugs_plugin.html) during Travis builds too, but findbugs works on the compiled bytecode, to find issues in code by analyzing the source code context.