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
watch for nsimplify making number 0
Change all the "assert Eq" tests to proper tests.
improved documentation of singular x**y cases
Remove duplicated docstring.
Merge pull request #2665 from jrioux/integrals

Remove duplicated docstring.
Merge pull request #2606 from zanzibar7/issue1015_patch

Raise correct error for issue 1015
Merge pull request #2670 from jrioux/tests

Change all the "assert Eq" tests to proper tests.
Merge pull request #2638 from smichr/4112

watch for nsimplify making number 0
Merge branch 'master' into singleton-docs-numbers
Reword some docstrings, mention that they are singletonized.
Add missing numbers singletons to sphinx docs