Fix to issue CAS-1249

Adds an optional parameter to AbstractInMemoryThrottledSubmissionHandlerInterceptorAdapter that configures the IPAddress Interceptors
to use the configured header instead of  the value of request.getRemoteAddr() for reading the client IP address.

Example configuration:
```
<bean id="throttleInterceptor" class="org.jasig.cas.web.support.InMemoryThrottledSubmissionByIpAddressAndUsernameHandlerInterceptorAdapter">
<property name="IPAddressParam"><value>X-Forwarded-For</value></property>
</bean>
```