Add function to check if a pdf depends on a given variable.
Add functions to change the limits of a pdf for a given variable and renormalize accordingly.

ALERT!: It is still very experimental, since the resulting pdf is scaled by a factor to do the
        renormalization, and it's not yet clear how to operate with pdfs that have been scaled.
Make sure the pdf always evaluates to a non-negative value, whatever the
        values of the Function objects are. Default to zero if the product
        of functions is negative.
Add function to generate toy MC from a Decay3Body. It requires c++0x.
Implementation of the algorithm to generate toys from a pdf expression.
        It requires the generate function to be implemented in all the
        models used in the expression.
Preliminary implementation of the generate function for a Gaussian. Still
        needs to take into account limits.
Prepare the makefile to use the c++0x implementation.
Make the example gaussian fit generate toy events if requested.
Define one single random engine that all models have to use.
Define a virtual default generate function for models that only throws an
        exception. Models have to override it with their specific implementation.
Add functions to evaluate the inverse of the erf function.
Add generate function for the Crystal Ball model.
Add test of the Crystal Ball generation and fit.
Add generate function for the exponential model.
Add test of the exponential generation and fit.
Add generate function for the double Crystal Ball model.
Add test of the double Crystal Ball generation and fit.
Add the implementation of the inverse of the complemented imcomplete gamma
        integral, necessary for the generation with an Argus distribution.
Make the generator engine a private member and add a seed setter and a
        couple of functions to generate flat and unbiased random numbers.
Add generate function for the generalized Argus model.