Add support for the native JSON column type as introduced in MySQL 5.7

[bjornpost]

"As of MySQL 5.7.8, MySQL supports a native JSON data type that enables efficient access to data in JSON (JavaScript Object Notation) documents."

• https://dev.mysql.com/doc/refman/5.7/en/json.html

This adds support for (un)mashalling JSON column types. Required >5.7.8 to run properly.

[bjornpost]

Travis fails because it runs the tests against 5.6. Not sure how to fix? We could exclude these 5.7+ specific tests from the default testsuite, but that feels a bit like cheating. Also: would be nice if I could access the MySQL version used from the JsonType. That way we could use a text type for <5.7.8, and json for 5.7.8 and up.

@lox @jud Thoughts?

[bjornpost]

ping @lox @Jud