trigsimp and simplify

- simplify sums of trigs to exponential form if that's shorter `cos(x) + I*sin(x)` -> `exp(I*x)` [issue 3821](https://code.google.com/p/sympy/issues/detail?id=3821)

- add a line to trigsimp's tree that allows it to reduce expanded `tan double-angle expressions` back to `tan` [issue 3826](https://code.google.com/p/sympy/issues/detail?id=3826)