dbi: Provide transaction support in the Connection base class

* Move postgres routines to Connection
* Remove default implementation from MySQL
* Add custom beginTransaction to SQLite

2013-08-25  Holger Hans Peter Freyther  <holger@freyther.de>

	* Connection.st: Add >>#beginTransaction,
	>>#commitTransaction and >>#rollbackTransaction in the
	base class.

2013-08-25  Holger Hans Peter Freyther  <holger@freyther.de>

	* Connection.st: Remove >>#commitTransaction and
	>>#rollbackTransaction.

2013-08-25  Holger Hans Peter Freyther  <holger@freyther.de>

	* Connection.st: Remove >>#beginTransaction,
	>>#commitTransaction and >>#rollbackTransaction.

2013-08-25  Holger Hans Peter Freyther  <holger@freyther.de>

	* Connection.st: Add >>#beginTransaction.
	* SQLiteTests.st: Use >>#beginTransaction and
	>>#commitTransaction in the testcase.