Add sql.DB to Postgres and Mysql structs to allow closing in Close()

[Kay-Zee]

ref #97

Closes the underlying sql.DB when Close() is called for the two Drivers that uses sql.Conn

Only noticed this behaviour for Postgres and Mysql.

Let me know if you're happy with the way the two errors are handled in Close()

[coveralls]

Pull Request Test Coverage Report for Build 180

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 46.813%

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Totals
Change from base Build 178:	0.0%
Covered Lines:	705
Relevant Lines:	1506

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💛 - Coveralls