Printing: print x**-1.0 differently from 1/x

`x**-1.0` and `x**-1` should print differently.  Fixed this and added
tests.  Pretty printing seemed to work already but tests added.
Issue #2860 is related and seems fixed; added tests.
On 02/09/14 13:32, Sergey B Kirpichev wrote:
> Ok.  Lets keep things as is.

Done, rebased.