Implement validation keywords for arrays (#18)

* Added structure for parsing minItems and maxItems in the SchemaInteger type

* Run validation against minItems and maxItems

* Check if minItems is greater than maxItems before returning a valid schema

* Minor cosmetic changes to some tests

* Added construction and validation tests for minItems and maxItems

* Added uniqueItems flag to arrays

* Simplified array instantiation, and fixed some test cases checking for wrong errors.

* Implemented 'contains' functionality

yaml-validator/src/error.rs

@@ -194,7 +194,8 @@ mod tests {
                   level2:
                     type: object
                     items:
-                      leaf: hello
+                      leaf: 
+                        notype: hello
             "#,
         );
 

yaml-validator/src/lib.rs

@@ -102,6 +102,14 @@ enum PropertyType<'schema> {
 impl<'schema> TryFrom<&'schema Yaml> for PropertyType<'schema> {
     type Error = SchemaError<'schema>;
     fn try_from(yaml: &'schema Yaml) -> Result<Self, Self::Error> {
+        if yaml.as_hash().is_none() {
+            return Err(SchemaErrorKind::WrongType {
+                expected: "hash",
+                actual: yaml.type_to_str(),
+            }
+            .into());
+        }
+
         let reference = yaml
             .lookup("$ref", "string", Yaml::as_str)
             .map(Option::from)