Improve Spring integration

Spring beans needed for BTM can be configured as follows:
```xml
	<bean id="dataSource" class="bitronix.tm.integration.spring.PoolingDataSourceFactoryBean">
		<property name="className" value="..." />
		<property name="uniqueName" value="..." />
		<property name="maxPoolSize" value="..." />
		<property name="etc" value="etc" />
		<property name="driverProperties">
			...
		</property>
	</bean>

	<bean id="transactionManager" class="bitronix.tm.integration.spring.PlatformTransactionManager"/>
```
The PoolingDataSourceFactoryBean correctly works together with Spring's lifecycle management.