Add checkbox in UI to observe resources

Conflicts:
	leshan-standalone/src/main/java/leshan/server/servlet/ApiServlet.java
	leshan-standalone/src/main/resources/webapp/index.html
	leshan-standalone/src/main/resources/webapp/partials/resource.html
Ensure uniqueness of Observations per ResourceObserver and observed
resource
Add button for observing resources making use of tooltips
add method getResourceRelativePath and notify observer only 204
response.
Add ObservationRegistry as contructor parameter for LwM2mServer.
Remove observation ID and ignore 204 response on observe Request.
Make EventServlet ThreadSafe. Remove Closing continuation on
unregestering client.
add glyphicons.
Add cancel observation, change Rest API to start observation.
Merge remote-tracking branch 'origin/master' into observe

Conflicts:
	leshan-core/src/main/java/leshan/server/lwm2m/LwM2mServer.java
	leshan-core/src/main/java/leshan/server/lwm2m/client/Client.java
	leshan-core/src/main/java/leshan/server/lwm2m/message/californium/CaliforniumBasedRequestHandler.java
	leshan-core/src/test/java/leshan/server/lwm2m/message/californium/CaliforniumBasedObservationTest.java
	leshan-core/src/test/java/leshan/server/lwm2m/message/californium/CaliforniumBasedRequestHandlerTest.java
	leshan-standalone/src/main/java/leshan/server/LeshanMain.java
Delegate creation of relative path to TargetSpec instance
Simplify Observation API
Use californium from central