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

fixes the bug @Noblebrown mentions and adds some tests for it.
I don't think we are fully consistent.  I generally preview my changes and fix before posting for review.  Once we agree on a coding standard (see #4), we will hopefully have standard configuration for IntelliJ to help make things consistent.  Would love to get your suggestions in the coding standard issue.