Support lazy referencing

this PR would make the snowcrash able to recognize references which has been defined after the location of the usage. closes issue #84. this PR is not yet ready to merge.
This is the same as https://github.com/apiaryio/snowcrash/blob/master/src/PayloadParser.h#L560-592, Can't we create another function in `PayloadParser.h` for this and use that?