add errors for empty arguments to min and max, for #2191
Merge branch 'master' of github.com:JuliaLang/julia into jb/minmax

Conflicts:
	base/array.jl
remove redundant exponent() doc
fix min and max over dim on empty arrays
fix some more empty min/max cases
error for findmin, findmax on empty. #2191
Merge branch 'master' of github.com:JuliaLang/julia