Use C++11 standard library to generate random numbers

[bcoconni]

This PR addresses issue #666 and allows removing the static method FGJSBBase::GaussianRandomNumber and its static member gaussian_random_number_phase.

The method FGJSBBase::GaussianRandomNumber is replaced by the C++ standard library class std::normal_distribution and all the other calls to the system function rand() have been replaced by the C++ standard library class std::uniform_real_distribution. The purpose of that is to use the same random number generator for both the gaussian and uniform distributions.

A new class JSBSim::RandomNumberGenerator has been created to centralize the random seed value and make sure all the random generators in JSBSim are referring to the same instance of RandomNumberGenerator so that calling FGFDMExec::Srand() (or setting the property simulation/randomseed) has a global effect over JSBSim random number generation.

There are 2 exceptions to this behavior:

1. The method FGFunction::makeRandomEngine which allows using specific instances of JSBSim::RandomNumberGenerator at the explicit request of the user (i.e. when specifying either time_now or a value to the XML attribute seed).
2. The dispersion feature of FGXMLElement which is now using a local instance of JSBSim::RandomNumberGenerator which seed is based on the system clock. The purpose of the code is that the dispersion will change at each execution of a script (currently the dispersion is identical at each execution which kind of defeats the purpose of testing how some parameters dispersion influences the script results).