Support null filters like [?(@foo == null)] and [?(@foo != null)]
Null handling tests.
Updated dependencies in pom.
Improved exception handling.
Merge pull request #31 from jochenberger/add-test-for-issue-29-part-1

add a test for what's described in comment #4 of issue 29
Reverted exception in ArrayIndexFilter and fixed tests.
Revert "Merge branch 'override-jsonproviderfactory-per-thread'"

This reverts commit bebdc09fc61168e0f3aef4a9e19f34c3f20497b4, reversing
changes made to 18ee9ff071384977fa5ebfde3705cb1c0a2efb20.
Merge remote-tracking branch 'upstream/master'