Support for getting the length of an array.

[transparentech]

This commit allows you to use ".length" in a JSONPath to get the length of an array. This is supported by the Goessner implementation (according to the JsonPath Evaluator), but seems to be lacking in the Jayway implementation. For example, "$.store.book.length" returns 4.

I'm new to the JsonPath code so there is probably a better way of doing this. Feel free to make suggestions for improvement or alternate implementation.

[kallestenflo]

Just tried it with the Goessner impl and as you say it works just fine. In my opinion it's a bit ambiguous, maybe more so in the java world. The Goessner implementation contains some features that I'm not sure about. Do you have a killer use case or is it just a nice to have?

[transparentech]

We're using JsonPath as part of a testing framework which gets JSON documents as input and verifies their contents. Essentially, we have a list of JsonPath Paths that we execute on the JSON document and if they match the expected results, the test passes. Being able to get the length of an array in the JSON document is extremely useful in verifying the contents of the documents.

[kallestenflo]

I will close this in favor of #103 that provides the same functionality but with a more generic approach.