avoid panic on tx.commit calls

[arxdsilva]

Hey guys,

While using this mock pkg I came across a commit panic when the mock tx was used to commit. I made a branch that shows how this panic occurs (here).

This PR does:

• makes Mock struct implement the driver.Driver interface
• registers a new driver "mock" on New
• returns a fake db tx on mock.GetTransaction
• implements mockConn that allows Mock{} to return a mocked connection

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