Make JdbcLog configurable to skip heartbeats

I think it'd be very useful to make it configurable whether or not JdbcLog prints heartbeats, just like ScreenLog does.
This may be an addition to issue 75 (to make names of JDBC tables configurable).

We took a stab at making JdbcLog more configurable, i'm attaching the patch file (apply in core/) that contains all the changes and a unit test.

We use Spring to configure our app (and QuickfixJ), and the relevant code to create a JdbcLog is:
<bean id="jdbcLogFactory" class="quickfix.JdbcLogFactory">
<constructor-arg>
<ref bean="sessionSettings"/>
</constructor-arg>
<property name="skipHeartbeats" value="true"/>
</bean>