Changed match func strategy; added tests

As it turns out, closures are very hard to validate without running them.
Since table-driven tests tend to rely on value types that can be
compared directly, using structs that adhere to a generic callback
interface is more work, but is more easily tested.

* Changed jsonpath match functions to structs with Call() methods
* Added tests to verify the generation of jsonpath QueryPath data
* Added tests to verify jsonpath lexer
* Fixed jsonpath whitespace handling bug
* Fixed numerous flaws in jsonpath parser