Implement barrier for nodes synchronization

  * Merge pull request. Closes #49.
  * Adds synchronization on FenixFramework.getBarrier() method (used by
    FenixFramework.barrier(...))
  * Minor fixes
Add properties to config [expectedInitialNodes, jGroupsConfigFile]

  * Infinispan-based backends (infinispan and ogm) now wait until the expected
    number of nodes are up before returning from the initialization process of
    the framework.  This allows for the use of GMU an Infinispan protocol that
    does not implement state transfer (yet?).  By default, the
    expectedInitialNodes=1, in which case no waiting is required
  * Uses JGroups to communicate between the nodes.