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.
@pksunkara it doesn't and it shouldn't since in here: https://github.com/alikh31/snowcrash/blob/alikh31/lazyreferencing/src/SerializeYAML.cc#L66 , `value` is empty and else condition happens.