Created the abstract TimeFactory class
Removed the standard constructor of the EvaporationPolicy class

Use only standard constructors if there really exist meaningful default values!
The PheromoneEvaporation class uses now a TimeFactory
Removed the need for Time::isInitialized()
Renamed toSeconds and toMilliseconds
Replaced Time::operator- with the new subtract method.

This will be nessecarry in the future when Time is only an abstract
pure interface
The class Time is now a pure virtual interface.
Implemented a LinearEvaporationPolicyMock which will ease future testing
Merge branch 'time' into develop

The Time class is now a pure virtual interface with two concrete
implementations: UnixTime and OMNeTTime (and of course a TimeMock for
testing)

Conflicts:
	tests/libara/core/reinforcement/LinearPathReinforcementPolicyTest.cpp