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
Remove unused class
API to receive asynchronous responses
Default LwM2m server will now wait until all retransmissions failed before timing out.
release 0.1.4
next release cyle
Started DTLS support.

For now a second port is bound and you are supposed to use an hardcoded
PSK key.

To come: API and UI for setting PSK.
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