add capabilities to defined custom serializer
add simple test with custom JSON serializer
(GH-1) Add new test file to the manifest list.
(GH-1) Make the JSON serializer test optional.

This saves us from having to add a hard dependency on a JSON module, or from
having to implement a self-contained serialization algorithm that would be used
for testing only.
(GH-1) Fix typo in argument names.
(GH-1) Formatting.
(GH-1) Reserve uppercase variables for constants.
Fix test name.
(GH-1) Add basic test for freeze().
(GH-1) Add basic test for thaw().
(GH-1) Verify that freeze() and thaw() use the custom serializer.
Clean up error messages.
(GH-1) Serializer arguments must be coderefs.
(GH-1) Add tests for serializer args to new().
(GH-1) Add examples to the POD of new methods.
(GH-1) Remove trailing spaces.
Add missing tests count.
Test requeue_count().
(GH-1) Use JSON::PP as an explicit fallback.

As @pstadt noticed, it has been included in core since perl v5.13.9. So it
should make a safe fallback in case JSON::MaybeXS is not installed.
Move acknowledgements to the changelog.