Support for iterating across (-a) empty objects/arrays. Fixed issue with nested iterations.

Changes:
- Added support for iterating across (-a) empty objects/arrays. Previously:
```
$ echo '[]' | jshon -a -t
parse error: bad json on arg 1, "-a"
```

  Now produces no output, as expected.
- Fixed bug that would occur with nested iterations. Previously:
```
$ echo '{"a":[1,2],"b":["3","4"]}' | jshon -a -a -t
number
number
string
string
parse error: bad json on arg 1, "-a"
```

  Now:
```
$ echo '{"a":[1,2],"b":["3","4"]}' | ./jshon -a -a -t
number
number
string
string
```