Implementation of equals and hashcode for fix fields.

In quickfixj, fields override the Equals and Hashcode methods which is often useful (I could give examples but I'm sure they are pretty obvious). Not having them is not really a problelm (since Dictionaries etc have constructors that take the IEqualityComparer to use), but for people that migrate from quickfixj (like me) and possible also for people that just assume that Equals on FIX fields "just works" this is confusing.

This patch implements a comparison of the tag number and the field value by overriding them in the FieldBase class.