Raise correct error for issue 1015

	When doing Limit((1+x/(n+sin(n)))**n,n,oo).doit()
	we raises an error instead of returning the wrong answer.

	Now, 1**oo == nan instead of 1.
also set test for 1**NaN == NaN
Added documentation of singletons for Pow

- also, minor optimization
Reformat Pow singleton definitions as table, exclude expr with nan
PEP8: space after '('
Merge pull request #5 from skirpichev/zanzibar7-pr2606

a variant for Pow docstring
improved documentation of singular x**y cases
Merge pull request #2606 from zanzibar7/issue1015_patch

Raise correct error for issue 1015