Datacenter: send ack for cloudlet submission when requested
FederatedDatacenter: send ack for cloudlet submission when requested; 
DatacenterBroker.java: avoid utilization of uninitialized VMs; removal of schedulingInterval field; schedule of internal events.

- TODO replace returning the field from Vm.getCurrentAllocatedMips() by a call to getCloudletScheduler().getCurrentMipsShare()

- Updated readme and description of the examples 

- Removed priority field from the Vm class

- Added a restriction to VmSchedulerTimeShared: each virtual PE of a VM must require not more than the capacity of a physical PE

- Fixed sngle threshold example

- Updated javadocs (authors and some class descriptions)

- Updated the readme files