Jsonmerge

* Add validatorclass argument to Merge constructor.
* Fix errors in reference resolution with jsonschema>=3.0.0.
* This release removes the requirement for jsonschema<3.0.0 in
setup.py.

* Fix exception on Python 2.7 when objects contain non-ASCII keys.
* Require jsonschema<3.0.0 in setup.py since some tests currently fail
with jsonschema 3.0.0 pre-releases.
* More descriptive error messages in ArrayMergeById strategy.

* Fix exception when additionalProperties keyword is a boolean value.
* Fix an issue with arrayMergeById strategy, where a wrong list item was
merged in some cases, or an "Unresolvable JSON pointer" exception
raised. See issue 31 on GitHub (thanks to fdearle).

* Fix "maximum recursion depth exceeded" in get_schema() with schemas
that have recursive references.
* Allow a merge strategy to be defined at the same level as "anyOf",
"allOf" and "oneOf" keywords (thanks to Henrique Cabral).
* Add reference to the offending JSON value in exceptions thrown by
jsonmerge.
* Add "discard" merge strategy.
* Raise SchemaError when encountering an unknown merge strategy name.

* More consistent handling of $ref references in get_schema(). Internal
references are now followed, but always kept unresolved in the final
result instead of replacing them with the referenced part of the
document. External references are never followed. This removes the
need to call resolve_refs() in custom merge strategies.
* Support for "oneOf" schema keyword.
* Fix compatibility with jsonschema > 2.4.0.
* Remove support for Python 2.6.

* Add "objClass" option for selection of an alternative class for
storage of JSON objects, like OrderedDict (thanks to Ray Plante).
* Deprecate jsonmerge use with Python 2.6.