Json method cleanup

[zslayton]

cc @alexcrichton, @arjantop

The new convenience methods on the Json class could be cleaned up a little bit:

• All of the methods use the corresponding Json enum variant in their name except for is_str and as_str, which refer to the Rust str type. These should be made is_string and as_string.
• Implement the is_* methods in terms of the as_* methods.
• find_path uses a fold/map implementation that does not short circuit in the event of an invalid path. If a path with 10 keys is provided but the first key does not exist, it will still go through the effort of generating all 10 Nones.

I'll take care of these, I just wanted to post them here in case people had other comments.