Remove android code / gradle build system / port tests to jUnit / additional tests

It would be really awesome if the community had an up-to-date version of cassowary for java. The most recently worked on [fork](https://github.com/ebolwidt/cassowary-java) does not appear to be maintained.

I've removed the android code and added it [to a different fork of this project that deletes all the cassowary code and depends on this project](https://github.com/alexbirkett/pybee-cassowary-java)

I've added support for the gradle build system. This is the default for Android Studio, my hope is that it's straightforward to import the project. You can select the java sdk instead of android if you wish since there are no android dependencies. This means tests can run on your machine using the jre rather than loading an APK on to an Android device, which is quicker! (by could possibly have different behaviour)

I've ported the test cases to jUnit and added some additional tests, some of which fail. I believe the defects will also affect your android project. I need to help to fix them. I'll raise separate issues about them.

I've updated the readme with info about how to depend on this project. Currently the artefacts are hosted on github but maybe we should consider trying to get them into maven central. / jCenter?