Resource domain support, plus configuration through a property file, plus unit tests.

I'll try to explain the rationale behind my changes.

We use a static resource domain for our files, so I needed to add support for that. Initially I just added a new context parameter, but then I realized we use different domains in our development, testing and production environments, so I needed more flexibility in the configuration of all options.

And I added property-based configuration (see [this issue](https://github.com/tatablack/wro4j-taglib/issues/1), the updated [README.md](https://github.com/tatablack/wro4j-taglib/blob/domain-support/README.md) file - or the code itself - for the loading sequence).

After that, since there were no unit tests, I had to add them too (my team would not have considered using an external library without tests).

I also updated the README file to reflect my changes. I'd be glad to update your companion project (https://github.com/Orange-OpenSource/test-release-wro4j-buildtime) too; we can discuss about it later, I suppose.

Let me know what you think, and thanks for your library, anyway..  :+1: