roots allows trig keyword: return cubic roots in terms of cos

This was motivated by a question at http://stackoverflow.com/questions/24740517/python-sympy-trigonometric-solution-of-cubic-equation.  A special class of cubics has real solutions that can either be expressed in terms of expressions containing sqrt(-1) -- which cancels out at evaluation -- or in terms of the transcendental functions `cos` and `acos`. Using the `trig` keyword allows the latter instead of the former (which is the default).