Added missing Field getters/setters (#82)

Author: vincentmarinelli-wk

src/main/java/io/frictionlessdata/tableschema/field/Field.java

@@ -551,14 +551,26 @@ public String getTitle(){
         return this.title;
     }
 
+  public void setTitle(String title) {
+    this.title = title;
+  }
+
     public String getDescription(){
         return this.description;
     }
 
+  public void setDescription(String description) {
+    this.description = description;
+  }
+
     public Map<String, Object> getConstraints(){
         return this.constraints;
     }
 
+  public void setConstraints(Map<String, Object> constraints) {
+    this.constraints = constraints;
+  }
+
     public URI getRdfType() {
         return rdfType;
     }