add tests to check that exceptions are thrown for wrong types

* Add JSONTEST_ASSERT_THROWS macro to test if an expression
  throws an exceptions.
* add JSONTEST_FIXTURE(ValueTest, typeChecksThrowExceptions)
Merge pull request #25 from cgilling/master

add tests to check that exceptions are thrown for wrong types

Nice!

For the record, I would have put the add-failure into the `try` block, for simplicity.