Positive handler

AskPositiveHandler iterates assumptions to find useful inequalities.
For example, ask(Q.positive(x), Q.is_true(x>1)) is True.

If this pull request is accepted, it could be easy to add many more without code duplication. Q.negative(expr) could call Q.positive(-expr), Q.is_true(StrictGreaterThen(a,b)) could call Q.positive(a-b) and so on.

I included one patch from https://github.com/sympy/sympy/pull/1890 because it is needed for some tests I added.
However, this pull request still work without 1890, only some tests would be marked @XFAIL.