Provide Conn type

[AlekSi]

Similary to reform.DB and reform.TX, we should provide reform.Conn type like database/sql.Conn.

This is very important for drivers that implement database/sql/driver.SessionResetter interface. With them, hacks like

// Use single connection so various session-related variables work.
// For example: "PRAGMA foreign_keys" for SQLite3, "SET IDENTITY_INSERT" for MS SQL, etc.
sqlDB.SetMaxIdleConns(1)
sqlDB.SetMaxOpenConns(1)
sqlDB.SetConnMaxLifetime(0)

no longer work.

Conn requires context, so #73 should be done first.

See also #151.