Probability distributions implemented in CloudSim replaced by similar functions from Apache math commons.

* Set the Java source version to 6 in the main pom.xml, for consistency

Performance improvement of the Logger. Ubiquitous string concatenation when the logger is off is now avoided.

* The minimal time between events is now configurable

Solved Issue 44: UtilizationModelPlanetLabInMemory: use a global constant to define the size of the data field: a new constructor for the classes, allowing definition of data size, was added.

Solved Issue 49: Wrong calculation of debt during migrationL: all references to debt from Datacenter and its subclasses were removed.