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fixed jcoap-example & jcoap-applications classpath to remain at Java 1.6
removed outdated gradle build to maintain consistency
added license information to maven pom of jcoap
removed coapcache test implementation

added CoapClientChannel.createRequest() signature to consume all mandatory information required for a sound request message.
Thus it is not possible to create an unfinished request with this call.
old createRequest(reliable?, CoapRequestCode); is marked as deprecated as it allows creating unsound requests.
reordered the parameters for better human readability

Added Method signature CoapMessage.setPayload(CoapData data) to soundly set the payload including corresponding media type.

Changed the CoapResource interface by adding functions to add and remove arbitrary tags
Changed BasicCoapResource to implement the interface changes. If a resource is set to be observeble a tag(flag) "obs" is added automatically
Changed the core resource to append arbitrary tags to the resources

further classes have been adapted to the changes mentioned above