Fix issue 3890: dsolve failing for an equality multiplied by -1
http://code.google.com/p/sympy/issues/detail?id=3890

One issue was the reordering or args, which should not happen
for Tuple instances, and another issue was the unneeded call
to piecewise_fold.
Add dsolve test for issue 3890.
Add a comment about the handling of Piecewise.
Wrap long lines.
Rewrite using expr.has(*symbols).
Merge pull request #2181 from jrioux/solvers

Fix issue 3890: dsolve failing for an equality multiplied by -1