Add tests and integrate Ivy

This pull request:

* replaces our build.xml with a modified version of the one from [skeeto/sample-java-project](https://github.com/skeeto/sample-java-project);
* integrates Ivy into the project for dependency resolution, which obviates the lib directory;
* borrows HttpClientMock from toopher-android and adds an injection site to ToopherAPI; and 
* adds [toopher/toopher-python](https://github.com/toopher/toopher-python)-inspired unit tests.