New refine entry for the atan2 function

This PR adds the function refine_atan2. Compared with  PR #8476 in this one the functions is more clearly written with consisten variable names y, x. The doc string function is also corrected. 
a reply to https://github.com/sympy/sympy/pull/8476#discussion_r20700373:
"I am not sure I understand your suggestion."

The point is: x could have explicit "-" sign (for example: `x = -z = Mul(-1, z)`, where Q.positive(z) is True).  In this case you might want move it out of atan arguments.