Add initial implementation of the sql driver

Fixes #6

Usage

	import _ "github.com/cznic/ql/driver"
Fix driverExec result
Fix #29.
Support DROP/CREATE TABLE IF [NOT] EXISTS.

Fixes #27
Add memory sql/driver. Fixes #28.
Resolve #31.
Fix Tx isolation vs nesting dilemma: choose isolation.
Tweak newdriverRows.
Add interface assert for driverConn.
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