Fix for Issue #34: Task.setPercentComplete() sets a String for a Double property

fixes the bug @Noblebrown mentions and adds some tests for it.
Using hamcrest provides more readability of the code in the first place. Since I dont give up hope we will have some more tests in here in the future. I thought of it as a good idea. Some more Information and other advantages may be found [here](http://stackoverflow.com/questions/1701113/why-should-i-use-hamcrest-matcher-and-assertthat-instead-of-traditional-assert).