Direct Derivatives Implementation.

This deals with the direct solving of differential equations of the form d(g(x) * F(x)) = 0 . 
As shown in issue http://code.google.com/p/sympy/issues/detail?id=3160 . 

I have added a new hint called 'direct' which directly integrates the function of the above mentioned form, without calling odesimp.

    from sympy import *
    from sympy.abc import x
    f = Function('f')
    d = Derivative(x*f(x) , x , x, x)
    dsolve(d , f(x))
    f(x) == C1/x + C2 + C3*x

I've also tried to make necessary changes to classify_ode , so that it returns 'direct' also.

    classify_ode(d , f(x))
    ('direct',)

Do review the code , and tell me wherever to change it.