Ensured all attributes are sealed

Attribute lookups are faster when they're `sealed`; this is considered a best practice from Microsoft's framework design guidelines.

Author: JeremyCaney

Ignia.Topics/AttributeSetterAttribute.cs

@@ -41,7 +41,7 @@ namespace Ignia.Topics {
   ///   </para>
   /// </remarks>
   [System.AttributeUsage(System.AttributeTargets.Property)]
-  sealed class AttributeSetterAttribute : System.Attribute {
+  public sealed class AttributeSetterAttribute : System.Attribute {
 
   } //Class
 

Ignia.Topics/Mapping/AttributeKeyAttribute.cs

@@ -22,7 +22,7 @@ namespace Ignia.Topics.Mapping {
   ///   decoupled from the source attribute key.
   /// </remarks>
   [System.AttributeUsage(System.AttributeTargets.Property)]
-  public class AttributeKeyAttribute : System.Attribute {
+  public sealed class AttributeKeyAttribute : System.Attribute {
 
     /*==========================================================================================================================
     | PRIVATE VARIABLES

Ignia.Topics/Mapping/InheritAttribute.cs

@@ -23,7 +23,7 @@ namespace Ignia.Topics.Mapping {
   ///   useful when an attribute is expected to be inherited by all child topics.
   /// </remarks>
   [System.AttributeUsage(System.AttributeTargets.Property)]
-  public class InheritAttribute : System.Attribute {
+  public sealed class InheritAttribute : System.Attribute {
 
   } //Class
 

Ignia.Topics/Mapping/MetadataAttribute.cs

@@ -19,7 +19,7 @@ namespace Ignia.Topics.Mapping {
   ///   value property) or a collection of the metadata (in the case of a collection).
   /// </remarks>
   [System.AttributeUsage(System.AttributeTargets.Property)]
-  public class MetadataAttribute : System.Attribute {
+  public sealed class MetadataAttribute : System.Attribute {
 
     /*==========================================================================================================================
     | PRIVATE VARIABLES