solve: simplier code

sp.solve does fine without symbols specified.  We should do what
they do unless the user wants specific symbols.
solve: add multiple solns test
solve: improve single/multiple solns and single/multi vars
solve: update docs
solve: clean up solve for 2*x test
solve: cite issue number by the FIXME
Merge pull request #173 from cbm755/solve_changes

Solve changes