Instead of IOException which has to be handled by the developer, an unchecked exception is sufficient and makes it easier to statically create fixtures used in more than one test case.

Also, the existing documentation was wrong, when the resource does not exist, fixture() threw an IllegalArgumentException (Preconditions.checkArgument) not an IOException as stated in the javadoc.
Merge branch '723_fixtures_unchecked_exception' of jangalinski/dropwizard into jangalinski-723_fixtures_unchecked_exception
FixtureHelpers#fixture() should throw IllegalArgumentException

The semantics of IllegalArgumentException are a better fit for those methods
than IllegalStateException.

> Thrown to indicate that a method has been passed an illegal or inappropriate argument.
Merge branch 'jangalinski-723_fixtures_unchecked_exception'

Fixes #723, closes #724