added stub and tests for a evaporation policy
added member (of type EvaporationPolicy) to class RoutingTable
added stub for class CubicEvaporationPolicy
removed exception 'NoRemainingPheromoneException' and changed return type of evaporate method of class EvaporationPolicy
added initializePheromone() function to class AbstractARAClient
small changes to class AbstractARAClient
The class EvaporationPolicy got a new method which checks if the
evaporation should be triggered. The routing table class in turn has a
method checkForEvaporation() which calls the corresponding check method
in class EvaporationPolicy and is called in methods which access the
routing table. However, the appropiate method for determining the passed
time since last access needs to be discussed in issue #4
fixed issue in deleting routing table
added checkForEvaporation method in class LinearEvaporationPolicy. Methods needs to be tested (and the initializiation of the member 'lastAccessTime' needs to be changed)
initializes last access times now with 0 instead of current time.
changed definition of function evaporate in EvaporationPolicy classes
reactivated unit test LinearEvaporationPolicyTest
added unit test for method checkForEvaporation() of class LinearEvaporationPolicy
Added class Time which provides methods for determining (system) time difference
removed old code in LinearEvaporationPolicy
Merge branch 'develop' into evaporation

Conflicts:
	Makefile
	omnetpp/OMNeTARAClient.cpp
small modifications to the unit tests for classes RoutingTable and BestPheromonePolicy. The unit tests now create an instance of LinearEvaporationPolicy which in turn is passed to the constructor of the RoutingTable. This is required to do so in order to set the interval for the checkForEvaporation method so high that it does not decrease the pheromone value of the test routing table entries.
added usleep to class LinearEvaporationPolicyTest
small changes to the unit test for class LinearEvaporationPolicy
made EvaporationPolicy a standard class over pure virtual interface. The class now provides an implementation for the checkForEvaporation() method since the 'logic' of the function is the same for every derived EvaporationPolicy (this also goes for the determineEvaporationFactor)