#69 cleanSession flag feature

Enhancement #69. Added checkbox under Preferences/Connections to allow the user to set cleanSession flag.

	modified:   src/main/java/st/alr/mqttitude/preferences/PreferencesBroker.java
	modified:   src/main/java/st/alr/mqttitude/services/ServiceBroker.java
	modified:   src/main/java/st/alr/mqttitude/support/Preferences.java
	modified:   src/main/res/layout/preferences_broker.xml
	modified:   src/main/res/values/bools.xml
	modified:   src/main/res/values/strings.xml