Create the solveset module

The commit implements the solveset module in an attempt to
fix the problems with the current solve function. The commit
roughly follows my GSoC proposal
https://github.com/sympy/sympy/wiki/GSoC-2014-Application-Harsh-Gupta:-Solvers
and more details can be acquired through the discussion on the github pull
request https://github.com/sympy/sympy/pull/7523
Create the solveset module

The commit implements the solveset module in an attempt to
fix the problems with the current solve function. The commit
roughly follows my GSoC proposal
https://github.com/sympy/sympy/wiki/GSoC-2014-Application-Harsh-Gupta:-Solvers
and more details can be acquired through the discussion on the github pull
request https://github.com/sympy/sympy/pull/7523
correct RootOf(...) in S.Reals
Add imageset.intersect(S.Reals) method