Random number generator + some tidying

Added a wrapper around C++ standard random number generator. All
test related .cpp files are moved into tests/ directory. Makefile
rewritten correspondingly. Executable binaries of the tests will
still be stored at the project's root directory, for convenience
of debugging.

	modified:   Makefile
	modified:   randomGenerator.cpp
	modified:   randomGenerator.h
	renamed:    testParam.cpp -> tests/testParameter.cpp
	renamed:    testPopulation.cpp -> tests/testPopulation.cpp
	modified:   tests/testRandomGenerator.cpp
	renamed:    testSpecie.cpp -> tests/testSpecie.cpp
	renamed:    testTotalPopulation.cpp -> tests/testTotalPopulation.cpp