Inner map can be pointer to map

[ianic]

I'm using go-simplejson in an application for finding differences between two maps. Go-simplejson is a great tool to ease navigation through maps. I find it useful that maps could have pointers to maps at some levels. When I found two same pointers I know that those levels are the same and don't need to recurse into to find differences. Helps a lot when finding differences in large similar maps.

[wpjunior]

I liked a lot this PR +1 =)

[antsbean]

I liked a lot this PR +1 =)