#183 - support YAML aliases in validation

1. Aliases are correctly dereferenced and expanded in-place by
SnakeYaml. 
2. Then we convert the object (usually it's a `Map`) to a string, which
is passed to Jackson's Mapper.

I hope to find a better way of transforming Snake-YAML results into a
`JsonNode`

com.reprezen.swagedit/src/com/reprezen/swagedit/editor/SwaggerDocument.java

@@ -397,7 +397,8 @@ private void parseYaml(String content) {
 
  private void parseJson(String content) {
  try {
- jsonContent = mapper.readTree(content);
+ Object expandedYamlObject = new com.fasterxml.jackson.dataformat.yaml.snakeyaml.Yaml().load(content);
+ jsonContent = mapper.readTree(mapper.writeValueAsString(expandedYamlObject));
  jsonError = null;
  } catch (Exception e) {
  jsonContent = null;