Add NArray#map alias for NArray#collect

[zaeleus]

Array#map was introduced in Ruby 1.6. This was likely because map is a more common term than collect. It would be appropriate to add an NArray#map alias for NArray#collect to be more consistent to Array#collect.

Patch here: https://gist.github.com/zaeleus/5156053

[masa16]

I have imported this patch. Thank you.