Add native JSON data type support, through a new adapter

[64kramsystem]

This PR introduces the adapter Mysql2JsonAdapter, which adds native support for the JSON data type.

The most immediate consequences are:

• the user doesn't need to specify anymore the JSON column data type;
• dumping a schema now supports the JSON data type.

Closes #6.

[coveralls]

Pull Request Test Coverage Report for Build 63

• 25 of 28 (89.29%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+3.6%) to 87.5%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/active_record/connection_adapters/mysql2_json_adapter.rb	22	25	88.0%

Totals
Change from base Build 61:	3.6%
Covered Lines:	49
Relevant Lines:	56

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💛 - Coveralls