Resolve function overloads for simple parameter types.

I've implemented a function overload resolution for simple parameter types. For example, consider an overloaded function f:

    void f(int, int);
    void f(double, double);

With this change it is possible to specify which overload to use in doxygenfunction:

    .. doxygenfunction:: f(int,int)

or

    .. doxygenfunction:: f(double,double)

If parameters are not specified in doxygenfunction, no overload resolution is performed as before.

Limitations: qualifiers (const, volatile) and complex declarators (pointers, references, etc) are not yet supported in parameter types, but should be relatively easy to add.

All the code that implements overload resolution is in DoxygenFunctionDirective.run. I'm not sure if it is the best place though.